Recently, I found myself speechless as I read through the annual report for Great Northern Iron Ore Properties (NYSE: GNI). I had been given a research report that rated the shares a buy and didn't mention anywhere that the trust will cease to exist on April 6, 2015. And yet there it was, clear as day, in the very first section of the company's 10-K.

This would be fine except that, considering the expiration date on the shares, there seemed almost no way to justify the share price at the time. My first thought was that investors must be getting very sloppy with their research. Then I started to wonder what else investors may not know about the companies that they're investing in or thinking about investing in. Companies like, say, Berkshire Hathaway (NYSE: BRK-A) (NYSE: BRK-B).

Reading through some of Berkshire's most recent Securities and Exchange Commission filings, here are a few things that you may have missed about Berkshire if you haven't bothered to crack open its filings.

The Berkshire Hathaway railroad company
We all know that the acquisition of Burlington Northern was a huge bite for Berkshire, but just how much does it change the company's business mix? During the third quarter, Burlington Northern contributed $1.1 billion to Berkshire's total operating income, as compared to $1.5 billion for the company's entire insurance group (GEICO, General Re, etc.).

Burlington Northern also dwarfed the other non-insurance businesses. The MidAmerican energy business registered $416 million in operating income, Marmon kicked out $212 million, and the "other businesses" category -- which includes a plethora of Berkshire-owned businesses from See's Candies to Fruit of the Loom and Dairy Queen -- totaled $844 million.

Warren Buffett is a business segment
Yes, I know, Warren Buffett is the chairman and CEO of Berkshire -- and he's just a person. But when you read through the Berkshire filings, Buffett comes off as if he's a business segment unto himself.

Here's what the quarterly filing has to say about Berkshire's insurance businesses:

Our management views insurance businesses as possessing two distinct operations -- underwriting and investing. Underwriting decisions are the responsibility of the unit managers; investing is the primary responsibility of Berkshire's Chairman and CEO, Warren E. Buffett. Accordingly, we evaluate performance of underwriting operations without any allocation of investment income.

This is interesting as it sets the insurance operations apart from competitors such as Allstate and Progressive (NYSE: PGR). While their respective chief investment officers, Judith Greffin and William Cody, perform (somewhat) similar functions at their companies, you won't find any of their names anywhere in their companies' most recent quarterly SEC filings. Even Tom Gayner at Markel (NYSE: MKL), a very sharp investor that many have likened to Buffett, doesn't get the same treatment.

I think this is notable as a reminder of how Berkshire has grown into a giant conglomerate that isn't completely reliant on the continued involvement of Warren Buffett. Sure, he may be better suited than most to invest the insurance float, and he pulls the trigger when new businesses are acquired, but the businesses that exist within Berkshire are independently operated and evaluated and should be able to continue clicking without the great Mr. Buffett.

Berkshire bonked by BP
After the BP (NYSE: BP) catastrophe in the Gulf of Mexico, Berkshire's reinsurance group ended up on the hook for some of the damages. Of the $158 million in catastrophe losses that Berkshire Hathaway Reinsurance Group recognized in the first nine months of 2010, most came from the Chilean earthquake in February and the BP oil spill.

A steal of an investment
When Berkshire agreed to invest a combined $8 billion in Goldman Sachs and General Electric (NYSE: GE) preferred shares that yielded 10% and included warrants to buy common stock, it made out like a bandit. Unfortunately, the big dividends from these companies may not last all that long, and Berkshire knows that investment opportunities like these don't come around every day. The company notes:

[O]ur investment in 10% GS Preferred may be redeemed at the option of Goldman Sachs at any time and our investment in 10% GE Preferred may be redeemed at the option of General Electric beginning in October 2011. Based on current market conditions, reinvestment of any redemption proceeds would likely generate significantly lower investment income in the future.

Not so keen on Nike
During the third quarter of last year, Berkshire reduced its investment in Nike by more than half. This is the kind of information that can be of note to both Berkshire shareholders -- as they watch how Buffett shapes the company's portfolio -- and Nike shareholders, as they are left to ponder why one of the world's pre-eminent investors shaved his stake so drastically.

Granted, this kind of information tends to be covered in a variety of sources when it's released, but how can you be sure that they're not skipping over a bit of the information that may be most informative to you? I'll tell you how -- by going directly to the filings yourself. Companies like Berkshire that have significant investment portfolios file a form 13F-HR that discloses information about the company's portfolio at the end of every quarter.

Keep digging
Certainly, there's much more to learn from a close read-through of Berkshire's filings, but these tidbits give you an idea of what you might uncover. I think there is great value in the news and analysis that you can find all over the Web (including on!), but I also think all investors stand to gain by digging into SEC filings to better know the company they're invested in.

Here's something you won't find in Berkshire's filings -- a free report from my fellow Fools identifying three stocks that Mr. Buffett wishes he could buy.

Berkshire Hathaway and Nike are Motley Fool Stock Advisor picks. The Fool owns shares of Berkshire Hathaway and Markel, which are Motley Fool Inside Value recommendations. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Matt Koppenheffer owns shares of Berkshire Hathaway and BP but does not own shares of any of the other companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.