Aon may have many more exchange enrollees than the government's group exchange signed. (LHP photo/Allison Bell)

Aon Hewitt is reporting that about 600,000 active employees and dependents of active employees actually signed up for major medical coverage through its private employer exchange system for 2014.

Aon Hewitt, an arm of Aon (NYSE:AON), reported two years ago that it was hoping to serve employers with 100,000 plan enrollees through its private exchange program for active employees in 2013. It announced a year ago that it was hoping to serve 600,000 this year.

All of the 18 large employers that used the private exchange program this year will be using it again next year, and rates for those employers will increase an average of about 5.3 percent in 2015, the company says. The company is not yet saying how many people it will enroll in 2015.

Aon Hewitt analysts are predicting that average rates for large self-insured U.S. employers will increase 6.5 percent to 8 percent next year, before taking benefits changes and plan design changes into account.

Because many exchange users chose to buy cheaper coverage, the employers that used the exchange ended up spending about $750 less per employee on coverage than originally predicted, Aon Hewitt says.

Aon Hewitt released its private group exchange figures as members of the House Small Business Committee were complaining at a hearing about their inability to get information about Patient Protection and Affordable Care Act (PPACA) small-group exchange plan enrollment from the U.S. Department of Health and Human Services (HHS).

ACASignups.net says state-based exchanges that have released group plan enrollment data have reported having about 72,000 group exchange plan enrollees.