Segments - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenue	$ 4,274	$ 4,243	$ 4,193	$ 4,119	$ 4,664	$ 4,697	$ 4,883	$ 4,646	$ 16,829	$ 18,890	$ 19,883
Margin	578	$ 561	$ 583	$ 581	664	$ 566	$ 673	$ 615	2,303	2,518	1,810
Goodwill and intangible assets, net	172				190				172	190	255
Total assets	6,787				7,154				6,787	7,154	8,471
Health Plans
Segment Reporting Information [Line Items]
Total revenue									16,815	18,471	19,352
Goodwill and intangible assets, net	172				190				172	190	212
Total assets	5,265				6,165				5,265	6,165	6,347
Other
Segment Reporting Information [Line Items]
Total revenue									14	419	531
Margin									0	43	29
Goodwill and intangible assets, net	0				0				0	0	43
Total assets	$ 1,522				$ 989				$ 1,522	$ 989	$ 2,124