Although the percentage of insurance companies that owe rebates this year is relatively small, about 14 percent, many giants of the industry are on the list. They include Aetna, Cigna, Humana and UnitedHealthcare. President Obama is highlighting the rebates as a tangible early benefit of the legislation; on the day the Supreme Court upheld the law as constitutional last month, he said millions of Americans would see rebates because their insurance companies had "spent too much on things like administrative costs and C.E.O. bonuses, and not enough on your health care." Self-insured employers, which cover more than half the nation's workers, are exempt from the new rule, as are Medicare and Medicaid.