Many of the largest carriers sell Medicare supplemental insurance. UnitedHealthcare, the health insurance unit of UnitedHealth Group, dominates the Medigap market because of its relationship with AARP.
More than 4 million seniors had a UnitedHealthcare Medigap plan in 2015, or about one-third of the entire Medigap population.
Blue Cross and Blue Shield insurers also hold a lot of power in their respective markets. Health Care Service Corp., the Blues carrier in five states, had more than 615,000 Medigap members last year. Publicly traded Anthem declined to disclose the exact number of Medigap enrollees, but industry analysts estimate Anthem covers around 850,000 members.
Aetna and Humana, which are trying to obtain regulatory approval for their pending merger, have a combined 725,000 Medigap enrollees. John Osborn, president of Osborn & Associates, said the Aetna deal has created anxiety in his state, where regulators recently called the merger anti-competitive.