Medical Claims and Benefits Payable - Medical Claims and Benefits Payable (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Other Liabilities Disclosure [Abstract]
Fee-for-service claims incurred but not paid (“IBNP”)	$ 1,444	$ 1,406
Pharmacy payable	150	126
Capitation payable	62	55
Other	325	267
Medical claims and benefits payable	$ 1,981	$ 1,854	$ 1,995	$ 1,961