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, Atmos Energy (NYSE:ATO).

Atmos Energy shares have outperformed the S&P 500 over the past quarter-century, and with less volatility:

Source: S&P Capital IQ.

Since 1987, shares have returned an average of 10.5% a year, compared with 9.7% a year for the S&P (both include dividends). One thousand dollars invested in the S&P in 1987 would be worth $19,200 today. In Atmos Energy, it'd be worth $24,700.

Dividends accounted for a lot of those gains. Compounded since 1987, dividends have made up 77% of Atmos Energy's returns. For the S&P, dividends account for 39% of total returns.

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

Source: S&P Capital IQ.

Some underperformance here. Since 1995, Atmos Energy's earnings per share have increased by an average of 3% a year, compared with 6% a year for the broader index.

What's that meant for valuations? Atmos Energy has traded for an average of 18 times earnings since 1987 -- below the 24 times earnings for the broader S&P 500.

Through it all, shares have been strong performers over the past quarter-century.

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

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.