Investing isn't easy. Even Warren Buffett counsels that most investors should invest in a low-cost index like the S&P 500. That way, "you'll be buying into a wonderful industry, which in effect is all of American industry," he says.

But there are, of course, companies whose long-term fortunes differ substantially from the index. In this series, we look at how individual stocks have performed against the broad S&P 500.

Step on up, UnitedHealth (NYSE: UNH).

UnitedHealth shares have simply demolished the S&P 500 over the last quarter-century:

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Source: S&P Capital IQ.

Since 1987, shares have returned an average of 24.1% a year, compared with 9.7% a year for the S&P (both include dividends). That difference piles up fast. One thousand dollars invested in the S&P in 1987 would be worth $19,200 today. In UnitedHealth, it'd be worth $990,300.

Dividends accounted for some of those gains. Compounded since 1987, dividends have made up about 5% of UnitedHealth's total returns. For the S&P, dividends account for 39% of total returns.

Now have a look at how UnitedHealth earnings compare with S&P 500 earnings:

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Source: S&P Capital IQ.

Huge outperformance. Since 1995, earnings per share have grown by an average of 19.3% a year, compared with 6% annual growth for the broader index.

What's that meant for valuations? UnitedHealth has traded for an average of 19.7 times earnings since 1987 -- a bit below the 24 times earnings average of the S&P 500.

Through it all, shares have been massive outperformers over the last quarter-century.  

Of course, the important question is whether that will continue. That's where you come in. Our CAPS community currently ranks UnitedHealth with a five-star rating (out of five). Do you disagree? Leave your thoughts in the comment section below, or add UnitedHealth to My Watchlist.

Fool contributor Morgan Housel doesn't own shares in any of the companies mentioned in this article. Follow him on Twitter @TMFHousel. Motley Fool newsletter services have recommended buying shares of UnitedHealth Group. Motley Fool newsletter services have recommended creating a diagonal call position in UnitedHealth Group. The Motley Fool has a disclosure policy. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.