McCutchen argued that US Airways had no right to get back more cash than he had received, and that US Airways had a responsibility to pay its fair share of the costs of the contingency fee.

A U.S. District Court judge granted summary judgment to US Airways. A three-judge panel at the 3rd U.S. Circuit Court of Appeals later threw out the District Court ruling, arguing that US Airways could not get a windfall by enforcing a reimbursement clause that would leave McCutchen with less than full payment for his medical bills.

When the Supreme Court took up the US Airways case, it said the key question was whether the 3rd Circuit had correctly held that Section 502(a)(3) of ERISA "authorizes courts to use equitable principles to rewrite contractual language and refuse to order participants to reimburse their plan for benefits paid, even where the plan's terms give it an absolute right to full reimbursement."

The Supreme Court majority held in the ruling issued Tuesay that the plan sponsor, US Airways Inc., should have to pay its fair share of the contingency fee.

Even though the plan documents said US Airways had the right to recover reimbursement for medical expenses if an injured enrollee won a lawsuit and recovered damages, the plan documents said nothing about how the plan would handle a recovery for attorney's fees, Justice Elena Kagan wrote in an opinion for the majority.

Normally, federal courts are not supposed to use "equitable law" doctrines — efforts to use traditional standards of fairness to do what seems right, rather than sticking rigidly to the letter of the law, or to the letter of a document — when dealing with ERISA plan matters.

But, because the US Airways plan documents involved in the McCutchen case say nothing about reimbursement for attorney's fees, the court can use the equitable law "common-fund" doctrine to handle the topic of allocation of responsibility for attorney's fees, Kagan said.

Under the common-fund doctrine, "a litigant or a lawyer who recovers a common fund for the benefit of persons other than himself or his client is entitled to a reasonable attorney's fee from the fund as a whole."

In US Airways, "without cost sharing, the insurer free rides on its beneficiary's efforts—taking the fruits while contributing nothing to the labor," Kagan wrote.

In that case, a beneficiary who wins reimbursement would be worse than a beneficiary who did nothing, Kagan observed.

If a plan is going to impose a provision that produces such a strange outcome, it must specifically provide for that in the contract, Kagan said.

Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Anthony Kennedy joined in the opinion.

Chief Justice John Roberts wrote a dissenting opinion on behalf of himself and justices Samuel Alito, Clarence Thomas and Antonin Scalia.

Representatives for the parties involved were not immediately available to comment on the ruling.

In 2012, when the Supreme Court was considering the case, the Blue Cross and Blue Shield Association argued in a brief that letting courts vary unambiguous plan terms in equitable relief actions would increase plan costs and undermine the purposes of subrogation.

Howard Shapiro, an ERISA litigation specialist at Proskauer Rose, agreed with the Blues in a comment on the case that, if the Supreme Court held that equitable principles could override unambiguous plan provisions, "the ruling might lead to a lack of standards and problems for other benefit plans and fiduciaries."