“In 2014 and 2015, nearly half of fully insured health plans reported underwriting losses,” the authors write. “While the number of health plans with losses moderated somewhat in 2016, the number and proportion of those with losses remains higher than pre-2014 levels.”

The largest health plans captured “a disproportionate, and growing” share of industry-wide underwriting gains, according to the report.

The three largest plans by revenue in every year from 2011 through 2016 were UnitedHealth Group, Kaiser Foundation Health Plan and Anthem. Their share of underwriting gains rose significantly, even as their share of enrollment and revenue moderated over the study period. In 2016, the top three generated 84 percent of all underwriting gains in the fully insured market, compared to 55 percent in 2011. Their share of enrollment declined to 30 percent in 2016, from 33 percent in 2011. Their revenue share also declined, to 34 percent in 2016 compared, to 36 percent in 2011.

The Top 10 plans by revenue varied somewhat from year to year between 2011 and 2016. Beyond the top three, the following five health plans remained in the Top 10 in each of the six years of Deloitte’s analysis: Aetna, Centene, Cigna, Health Care Service Corp., and Humana.

The vast majority of health plans in the U.S. fully-insured market are relatively small, with more than 70 percent posting annual revenue of less than $2 billion, according to the report. The financial performance of most of these smaller health plans is close to break-even.

The study also found that for-profit health plans grew faster and posted significantly higher margins than not-for-profit health plans. For-profit plans saw revenue jump by 60 percent between 2011 and 2016, while not-for-profit plans had a revenue increase of 49 percent during the same period. For-profit plans had an average margin in 2016 of 2.5; the non-profit margin was 0.8 percent in 2016.

Lastly, the study revealed state insurance markets experienced increased volatility. There was a marked increase in the number and variety of states with unfavorable swings in insurance market performance, with the performance of health insurance markets in many states deteriorating significantly.

“Our study shines a brighter light on the many U.S. health plans that have struggled financially, particularly since 2014, in a disrupted and highly competitive market,” the authors write.

“Many -- but not all -- of the health plans posting underwriting losses are smaller-scale companies, often nonprofits, which lack the financial resources to withstand many more ‘bad years,’” they add. “Many of these health plans play critical roles in their local communities and health care ecosystems; their financial prospects warrant continued attention.”