In a statement, Humana applauded the ruling, saying it provides security for consumers nationwide who get their coverage through an exchange-based plan.

“The Supreme Court ruling upholding subsidies means that people who currently receive a subsidy—whether they're enrolled in coverage through a state-based or federal exchange—will continue to receive the extra help,” Humana said.

The Supreme Court ruled 6-3 that the subsidies are legal on the federal exchange, upholding a major tenet of Obamacare that enables millions of Americans to keep the tax subsidies that help them buy and afford health coverage under the law. A lot was at stake in the case: An estimated 6.4 million consumers in 34 states currently receive subsidies through the federal exchange to help them pay for health coverage.

“As a result of the Supreme Court decision, a great deal of uncertainty regarding these polices has been avoided,” Moody's wrote in its analysis. “Had the Court found the subsidies to be illegal, a substantial number of these individuals would likely have canceled their polices either because they could no longer afford them or because they considered the polices not worth continuing at the full premium required.

“Since this latter group would most likely consist of healthier individuals, not only would insurers' revenue and membership have shrunk, but they would have been left insuring a less healthy population. Because insurance companies cannot change the premium rates for the remaining portion of 2015, there would likely have been financial losses on these policies.”

Still, Moody's predicted, PPACA will continue to present challenges to carriers from provisions of the law that have not yet been implemented, including the Cadillac tax. Preliminary rates showing premium increases also present a problem for 2016.