NEW YORK — UnitedHealth Group is buying surgical center operator Surgical Care Affiliates for about $2.3 billion in a cash-and-stock deal that will add to its outpatient holdings.

The Minnetonka based health insurer will add Deerfield, Illinois-based Surgical Care to its Optum health services unit, which has 20,000 affiliated physicians and hundreds of facilities. Surgical Care operates 205 surgical facilities and partners with about 3,000 physicians.

The acquisition will be funded with between 51 percent and 80 percent of UnitedHealth stock. The remainder will be in cash.

UnitedHealth Group Inc. is the nation’s largest health insurer. Its core insurance business helped swell profit 23 percent during its third quarter, while prompting a boost in its outlook.