UnitedHealthcare is in hot water again as the insurance giant grapples with a reported government investigation of its Medicare billing practices, pursues employee buyouts and potential layoffs, and clashes publicly with billionaire Bill Ackman.

Those developments in recent days extend a tumultuous past year for its parent company, UnitedHealth Group, marked by the killing of a top executive, a costly cyberattack against its subsidiary and high medical costs in its insurance arm. UnitedHealth Group is the biggest health-care conglomerate in the U.S. based on revenue and its more than $420 billion market cap, and UnitedHealthcare is the nation’s largest private insurer.

Shares of UnitedHealth Group have tumbled roughly 23% over the last three months.