Medical Claims and Benefits Payable - Incurred Claims and Allocated Adjustment Expense (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Product Information [Line Items]
Incurred Claims and Allocated Claims Adjustment Expenses	$ 13,905	$ 15,137	$ 17,073
Incurred Claims and Allocated Claims Adjustment Expenses	46,125
Total IBNP	1,391
Benefit Year 2017
Product Information [Line Items]
Incurred Claims and Allocated Claims Adjustment Expenses	16,704	16,728	$ 17,037
Total IBNP	$ 18
Cumulative number of reported claims (in claim) | claim	119,000,000
Benefit Year 2018
Product Information [Line Items]
Incurred Claims and Allocated Claims Adjustment Expenses	$ 15,245	$ 15,478
Total IBNP	$ 25
Cumulative number of reported claims (in claim) | claim	110,000,000
Benefit Year 2019
Product Information [Line Items]
Incurred Claims and Allocated Claims Adjustment Expenses	$ 14,176
Total IBNP	$ 1,348
Cumulative number of reported claims (in claim) | claim	93,000,000