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In a few states, the average cost of silver-level plans even dropped: Minnesota (-15 percent), North Dakota (-9.8 percent), Alaska (-22.5 percent) and Arizona (-2 percent)

Several states only experienced modest increases for silver-level plans: Oklahoma (5.1 percent), South Dakota (8.6 percent) and Vermont (0.2 percent).

However, the great majority of states saw major price hikes. In Iowa, the average silver-level premiums more than doubled (117 percent increase), from $326 to $695. Utah, Virginia, Wyoming, New Mexico, Georgia and Kentucky all saw prices go up more than 50 percent.

The study was based on what a 40-year-old nonsmoker would pay for the plans.

Some of the largest increases came in markets that had some of the lower rates last year. They may have been correcting for premiums that were too low to yield a profit. However, Iowa went from being in the middle of the pack to having the second-highest average rates, behind only Alaska ($698) and Wyoming ($860).

The study attributed the increases largely to political decisions in Washington D.C. One of the notable ones came last year when the Trump administration stopped reimbursing insurers for the cost-sharing reductions that they are required to provide low-income ACA customers.

Although the rates were set before the individual mandate was repealed in December, insurers had good reason to believe that the repeal was coming and likely priced their plans in anticipation. The reduction in the enrollment time period and a major decrease in marketing of ACA plans also led insurers to believe that enrollment would be low, particularly among young and healthy people.

Although enrollment turned out to be surprisingly strong in December, insurers are still nervous about how the repeal of the mandate and political uncertainty about the future of CSRs will affect the ACA marketplace in the long-term.