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It's incredible to think about just how far the Dow Jones Industrial Average (DJINDICES:^DJI) has come in such a short amount of time. Less than eight years ago, the Dow was mired in its worst bear market in multiple generations and was struggling to hold the 6,500 mark. Today, the Dow sits within striking distance (just 82 points) of 20,000, a mark that Wall Street pundits hold in high regard. That's a compound annual growth rate since March 2015 of more than 15%. For added context, stocks have historically returned about 7% annually, inclusive of dividend reinvestment.

Even though Dow 20,000, should it be hit, is nothing more than an arbitrary number that Wall Street has pulled out of thin air to celebrate -- (the Dow has hit 17 new all-time highs since the election, and each one is just as impressive as Dow 20,000 in their own right) – the Dow's march higher does offer a ray of sunshine. For starters, Dow 20,000 would probably represent a clear and defining closure of the Great Recession. The simple fact that the Dow blew through its pre-Great Recession highs in 2013 should have been enough to appease Wall Street, but it's clear that pundits are looking for added validation in the form of Dow 20,000 being hit.

Also, the Dow's continuing march higher, along with the Federal Reserve raising its benchmark rate for only the second time in a decade, suggests that the U.S. economy is thriving without training wheels for the first time in a long time. The unemployment rate is at its lowest level since August 2007, and GDP growth for the third quarter came in at its fastest quarterly pace (3.5%) in two years.

Lastly, the Dow's run to nearly 20,000 vindicates long-term investors who've held throughout the market's peaks and troughs.

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You'll never guess who's selling into Dow 20,000

However, not everyone is necessarily ringing the bell on Dow 20,000. A surprising group of "investors" has actually been ringing the register and selling into the recent rally.

As reported earlier this week by CNBC, company insiders have been dumping their own stock at a seemingly unprecedented rate since the election. According to proprietary research from InsiderScore.com, which analyzed the buying and selling activity of Wilshire 5000 insiders, there has been considerably more selling than buying among insiders since the election, especially in the banking, energy, and industrial sectors. This last point is of particular interest because these sectors have received the biggest "Trump bump" given the president-elect's plans to deregulate the banking industry, promote domestic drilling for the energy sector, and implement a 10-year, $1 trillion infrastructure spending plan for the industrial sector.

Additionally, Vickers Weekly Insider data found that there were nearly five insider sales for every insider purchase last week, which is nearly double the 2.5-to-1 ratio the report indicates is usually a bearish sign. Remember, insiders have a better understanding of the company they run and own stock in than Wall Street analysts, meaning if shareholders see insiders dumping, they could assume there's something wrong and head for the exits themselves.

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Should you really be worried?

Of course, there's a flip side to insider selling, too. We're at the end of the year, meaning some insiders may be selling for tax purposes. Considering that many high-ranking executives are primarily compensated in stock and options as opposed to salary and bonuses, it sometimes means that insiders need to sell stock in order to generate cash to cover their tax bills. If any of the companies you hold have seen an uptick in insider selling, you may want to check previous years to see if insiders made similar sales in December. If so, it could indicate a need to raise cash for tax purposes and not weakening insider sentiment.

It's also possible that some of the recent selling could have to do with insider portfolio rebalancing. As noted above, most insiders, especially CEOs, are compensated in stock and options as opposed to salary and bonuses. Giving leadership stock and options is a good way of tying their interests to that of shareholders. However, giving CEOs and insiders stock and stock options can also lever their portfolios too heavily to their own companies. Even CEOs want their investments to be diversified beyond just their own stock, so we could also be witnessing some degree of diversification ongoing.

Finally, consider whether the shares sold by insiders were tied to stock options. Remember, options have an expiration date by which they need to be executed, otherwise they expire worthless. Sometimes, insiders are selling stock of recently executed options in order to avoid having their options expire worthless. This, too, shouldn't be a cause for concern.

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Stick to your game plan

Regardless of whether a large uptick in insider selling is benign or representative of a genuine shift in insider sentiment shouldn't be of much consequence to investors or their game plans. In the end, as long as you buy and hold high-quality companies over the long term, the data suggests you're probably going to come out ahead.

Since 1950, the S&P 500 (SNPINDEX:^GSPC) has undergone 35 stock-market corrections of 10% or more, when rounded to the nearest integer, or about one every two years. In other words, corrections are actually somewhat common.

However, what's even more common are bull-market rallies. Over those past 66 years, we've spent considerably more days (almost 2-to-1) in a bull market as opposed to a bear market according to data from Yardeni Research, which is a testament to the patience of long-term investors. If the stock market does indeed gain 7% historically, then investors could see their money double about once every decade. Those are good odds, and a great reason for investors to stick to their game plans even with the Dow approaching 20,000.

Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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