A sagging economy and continually rising premiums contributed to a significant erosion of employer-sponsored health insurance during the past decade, according to a new report from the Robert Wood Johnson Foundation.

The state-by-state analysis found that employer-sponsored coverage fell 10 percent between 1999/2000 and 2010/2011. There was "substantial state-level variation." At 15 percent, Michigan saw the biggest drop. Meanwhile, Nebraska saw just a 4.3 percent drop. Alaska, Massachusetts and North Dakota had stable rates.

Several factors contributed to the decline, according to RWJF: decreases in the overall employment level, decreases of employees in firms that offer coverage, decreases in the number of workers eligible for employer-sponsored coverage and decreases in the number of enrolled dependents.

The most significant hit occurred in firms with 50 or fewer workers, falling to 37.5 percent, down from 47.2 percent.

RWJF officials say that the report should serve as a baseline for measuring the impact of health insurance exchanges, which officially take hold in January. In a related survey, the International Foundation of Employee Benefit Plans found that the vast majority of employers are developing strategies to deal with the ACA. Less than 1 percent said that they would discontinue coverage in 2014.

Here are some other key takeaways from the RWJF study:

  • New Hampshire (73.8 percent) had the highest rate of employer-sponsored coverage; New Mexico (48 percent) had the lowest rate.
  • More than half — 51.4 percent — of people with employer-sponsored coverage are dependents.
  • Small firms offering coverage fell from 67.7 percent to 56.3 percent.
  • The average total annual premium for single coverage at all firms offering coverage more than doubled, rising from $2,490 to $5,081.
  • The average employee premium rose to 20.8 percent, up from 17.5 percent.