The leader of the nation's largest health insurer warned Thursday not to assume widespread participation from his company in part of health care overhaul's coverage expansion that unfolds later this year. UnitedHealth Group Inc. CEO Stephen Hemsley told analysts the insurer's involvement in online exchanges that are expected to help millions buy coverage will depend on whether it's financially viable for the company. "We will only participate in exchanges that we assess to be fair, commercially sustainable and provide a reasonable return on the capital they will require," he said.