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

May 13 (UPI) -- UnitedHealth Group announced Tuesday CEO Andrew Witty will step down, citing "personal reasons." Witty will leave the role of CEO and be replaced by Stephen J. Hemsley, effective immediately.

Former UnitedHealth Group chief executive officer Stephen Hemsley to replace Andrew Witty. Here's what we know.

Witty’s departure was surprising, though it follows a sharp downturn in the healthcare juggernaut’s financial outlook. He is being replaced by Stephen Hemsley, the chair of UnitedHealth’s board and a previous chief executive.

Shares of UnitedHealth Group tumbled more than 15% in morning trade on Thursday after a report said that the Justice Department is investigating the health insurer for potential fraud.

UnitedHealth Group CEO Stephen Hemsley, who replaced Andrew Witty, said he has the right strategy in place “for the era ahead.”