Stephen Hemsley ran UnitedHealth Group for more than a decade. It's a different company after four years with Andrew Witty in charge.

Even UnitedHealth will struggle to overcome parasitic medical costs that Warren Buffett once called a tapeworm eating away at U.S. economic competitiveness. The $300 billion healthcare conglomerate reinstalled Chairman Stephen Hemsley as CEO and yanked its financial guidance.

Former CEO and current chairman Stephen Hemsley is stepping back into the top job following the abrupt resignation of Andrew Witty, UnitedHealth said.

UnitedHealth Group CEO Andrew Witty abruptly stepped down from the health insurance and care conglomerate on Tuesday, citing personal reasons, capping the end of a tumultuous four-year tenure that included the killing of the company’s top insurance executive .

Shares of UnitedHealth Group, Inc. (UNH) fell over 17% on Tuesday after the health insurance giant said CEO Andrew Witty would step down for personal reasons, with chairman and former chief executive Stephen Hemsley taking his place.