Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Schedule of Premium Revenue by Health Plan Type [Line Items]
Maturity period	10 years
Average maturity period	10 years
Interest receivable	$ 11	$ 12
Medical cost floors and corridors	74	74
Amounts due government agencies	777	664
Risk adjustment payable	456	368
Risk adjustment receivable	76	63
Risk adjustment, net payable	380	305
Risk adjustment, net payable, current year	80
Risk adjustment, net payable, prior year	300
Receivables from state and federal government agencies	1,603	1,406
HIF liabilities	271
Medicaid Expansion
Schedule of Premium Revenue by Health Plan Type [Line Items]
Amounts due government agencies	$ 69	$ 69