The number of employers participating in a health insurance exchange that Aon Hewitt launched earlier this year will leap six-fold in 2014, while the number of employee enrollees will triple, a reflection of the burgeoning employer interest in private health insurance exchanges.

In all, 18 employers, with 330,000 employees, will participate in the exchange in 2014. That compares with three employers — including U.S. employees of Aon Hewitt parent Aon P.L.C., Darden Restaurants Inc., the Orlando, Fla.-based restaurant chain, and Sears Holding Corp. of Hoffman Estates, Ill. — who offered coverage through the Aon Hewitt exchange this year.

Participating employers, all of whom have at least 5,000 employees, represent a wide range of industries, including retail and wholesale trade, business and professional services, hospitality, health care, technology, communications, financial, and manufacturing, Aon Hewitt said Wednesday.

“We are continuing to invest in the health of our team members and their dependents through expanded choices to personalize their health care coverage in a competitive environment, giving our diverse workforce the flexibility they need to meet their health care needs,” Kathleen Wilson-Thompson, senior vice president and chief human resources officer at Deerfield, Ill.-based Walgreen Co., said in a statement. Walgreens is one of the 15 employers joining the Aon Hewitt exchange in 2014.

The design of private insurance exchanges, such as the Aon Hewitt exchange, is appealing to both employers and employees, experts say.

The exchange model deploys a defined contribution approach in which employers agree to provide a fixed premium contribution with employees paying more or less for their share of the total premium depending on the level of coverage they choose.

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Through that approach, an employer can cap what it will pay for health care plan coverage for its employees.

In addition, the role of the employer as a health care plan sponsor becomes much more limited, with exchange insurers handling claims and the exchange vendor negotiating premium rates with insurers and assisting employees with questions they have.

For employees, the approach typically means a much wider choice of insurers and plan designs than they previously had.

In the Aon Hewitt exchange, about 20 different national and regional insurers — including Aetna Inc., which will be joining the exchange in 2014, Health Care Service Corp., which operates Blue Cross/ Blue Shield plans, and UnitedHealthcare Inc.— offer coverage.

Employees are able to choose from five different plan designs, including consumer-driven health care plans.

That range of plan design choices is appealing to employees, notes Ken Sperling, Aon Hewitt national exchange strategy leader in Norwalk, Conn.

“Employees, especially lower-income employees, appreciate the wide choice in plan design,” he said.

For example, some lower-paid employees, who may have little disposable income, would rather have the predictability through payroll deduction of a higher but fixed premium contribution than exposure to a high and uncovered medical expense, Mr. Sperling said.

Still, on average, employees moving to exchanges are more likely to select less generous plans compared with their prior coverage, Aon Hewitt found in an analysis of employee behavior in its exchange.

According to the analysis, 42% of employees selected a less generous plan compared with what they had previously, 32% selected a plan similar to what they had previously, and 26% opted for a richer plan.