Shares of UnitedHealth Group are surging before the market open Friday as Warren Buffett's Berkshire Hathaway disclosed that it recently purchased shares of the beleaguered insurer.

TheStreet. Warren Buffett is arguably the most successful investor of all time. He's amassed an amazingly massive net worth by buying and usually holding stocks for the long haul. Trained under legendary value fund investor Benjamin Graham,

Buffett owned UnitedHealth shares before selling his stake in 2010, though his Berkshire Hathaway recently acquired shares.