Segments - Reconciliation of Gross Margin to Consolidated Income (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 margin	$ 578	$ 561	$ 583	$ 581	$ 664	$ 566	$ 673	$ 615	$ 2,303	$ 2,518	$ 1,810
Add: Other operating revenues	$ 4,274	$ 4,243	$ 4,193	$ 4,119	4,664	4,697	4,883	4,646	16,829	18,890	19,883
Less: Other operating expenses									(15,785)	(17,744)	(20,438)
Less: loss on sales of subsidiaries, net of gain					$ (52)	$ 37	$ 0	$ 0	0	(15)	0
Operating income (loss)									1,044	1,131	(555)
Less: other expenses, net									72	132	57
Income (loss) before income tax expense (benefit)									972	999	(612)
Health Plans
Segment Reporting Information [Line Items]
Add: Other operating revenues									16,815	18,471	19,352
Other
Segment Reporting Information [Line Items]
Total margin									0	43	29
Add: Other operating revenues									14	419	531
Operating Segments
Segment Reporting Information [Line Items]
Total margin									2,303	2,518	1,810
Operating Segments | Health Plans
Segment Reporting Information [Line Items]
Total margin									2,303	2,475	1,781
Operating Segments | Other
Segment Reporting Information [Line Items]
Total margin									0	43	29
Other Operating
Segment Reporting Information [Line Items]
Add: Other operating revenues									621	871	508
Less: Other operating expenses									(1,880)	(2,243)	(2,873)
Less: loss on sales of subsidiaries, net of gain									$ 0	$ (15)	$ 0