Hospital (Photo: Thinkstock) (Photo: Thinkstock)

UnitedHealth Group Inc., the biggest U.S. health insurer, agreed to renew its contract with a large hospital staffing company after a months-long stand-off that threatened to put millions of people at risk of surprise medical bills.

The contract between UnitedHealth’s insurance unit and Envision Healthcare, owned by private-equity giant KKR & Co., was set to expire Jan. 1. The companies announced the deal in separate press releases Tuesday morning. Representatives for UnitedHealth and Envision both declined to comment on details of the new agreement.

(Related: UnitedHealth Could Break Up With a Big ER Doctor Employer)

The deal will keep Envision’s 25,000 clinicians in-network for UnitedHealth’s members, which include 27 million commercially insured people. It will also avert the threat of unexpected out-of-network hospital bills going to UnitedHealth customers who get unwittingly treated by Envision’s clinicians in emergency rooms, in surgery or in other settings in which they don’t necessarily choose their doctors.

Read more: An insurance dispute threatens patients with surprise bills

Envision sued UnitedHealth earlier this year and said that the insurer wasn’t paying it enough under an earlier agreement. UnitedHealth countered that Envision was trying to improperly boost profits without properly giving notice of price hikes. The dispute went to arbitration.

Envision has been criticized for billing patients when out-of-network insurers decline to pay the full price of its services. The practice, known as balance billing, has drawn scrutiny from regulators and Congress, and state and federal lawmakers have attempted to restrict it.

— Read Envision to Merge With AmSurg to Create Health Services Gianton ThinkAdvisor.

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