America's largest health insurers are seeing unprecedented growth in their Medicare Advantage businesses.
Here is the Medicare Advantage enrollment Aetna, Anthem, Cigna, Humana and UnitedHealthcare recorded as of Dec. 31, 2018, ranked from highest to lowest*:
1. UnitedHealthcare: 4.9 million
2. Humana: 3.1 million
3. Aetna, owned by CVS: 1.8 million
4. Anthem: 1 million
5. Cigna: 436,000
*Enrollment numbers are from insurers' most recent financial documents