The U.S.’s biggest health insurer, UnitedHealth Group Inc., will buy Surgical Care Affiliates Inc. for about $2.3 billion, adding an outpatient surgery chain to its growing health care-delivery business. UnitedHealth will pay $57 a share, with with 51 percent to 80 percent of that in stock and the rest in cash, the companies said in a statement.

UnitedHealth Group Inc. agreed to buy Surgical Care Affiliates Inc. for about $2.3 billion in cash and stock to add an outpatient surgery chain that will help the biggest U.S. health insurer diversify its business. UnitedHealth is paying $57 a share, with 51 percent to 80 percent of that in stock and the rest in cash, the companies said in a statement Monday.