Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
Schedule of Premium Revenue by Health Plan Type [Line Items]
Medical cost floors and corridors		$ 95			$ 103
Amounts due government agencies		612			967
Risk adjustment payable		285			466
Risk adjustment receivable		76			34
Risk adjustment, net payable		$ 209			432
Maturity period		10 years
Adoption of new accounting standard				$ 85		$ 6
Receivables from state and federal government agencies		$ 1,280			1,330
Retained Earnings
Schedule of Premium Revenue by Health Plan Type [Line Items]
Adoption of new accounting standard				85		$ 7
Retained Earnings | Accounting Standards Update 2016-02
Schedule of Premium Revenue by Health Plan Type [Line Items]
Adoption of new accounting standard				$ 85
CMS Subsidies
Schedule of Premium Revenue by Health Plan Type [Line Items]
Reimbursement	$ 5		$ 81
State and Federal Government Agencies | Accounts Receivable | Customer Concentration Risk
Schedule of Premium Revenue by Health Plan Type [Line Items]
Concentration risk, percentage		70.00%
Receivables from state and federal government agencies		$ 970
Medicaid Expansion
Schedule of Premium Revenue by Health Plan Type [Line Items]
Amounts due government agencies		$ 78			$ 87