The U.S.’s biggest health insurer, UnitedHealth Group Inc., will buy Surgical Care Affiliates Inc. for about $2.3 billion, further diversifying by adding an outpatient surgery chain.

The two companies previously worked together as partners. 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. The price is a 17 percent premium to Surgical Care’s closing value Friday.

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