UnitedHealth Group will promote one of its top insurance executives to replace Brian Thompson, the slain CEO of its UnitedHealthcare arm. The company said Thursday that Tim Noel will become the next UnitedHealthcare leader. Noel most recently led UnitedHealthcare’s Medicare and retirement business. He joined the company in 2007. UnitedHealthcare is the nation’s largest insurer, covering more than 49 million people.