UnitedHealthcare said on Thursday that it has agreed to take over almost 1 million health insurance customers of the Principal Financial Group, which is exiting that business.

UnitedHealthcare, which is part of Minnetonka-based UnitedHealth Group, has a national book of business that covers the health insurance spectrum. Des Moines-based Principal offers group plans in the central United States.

It plans to continue offering life insurance, dental, disability, vision and wellness programs.

As of the end of 2009, Principal had about 996,000 members enrolled in its medical plans. Customers will have the option to transfer to UnitedHealthcare upon renewal, so the addition of new members to UnitedHealthcare will be staggered.

About 381,000 of these members are risk-based, with bigger margins for the insurer, and 615,000 are fee-based.