Segment Information - Special Items (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Before-tax [Abstract]
Transaction-related costs									$ 852	$ 126	$ 166
Charges associated with litigation matters									25		40
Selling, general and administrative expense, measurement period adjustment									2	(56)
Total charges (benefits) associated with U.S. tax reform									2	(56)
Debt extinguishment costs						$ 321			0	321	0
Long-term care guaranty fund assessment										129
Risk corridor allowance											124
After-tax [Abstract]
Transaction-related costs	$ 402	$ 108	$ 109	$ 50	$ 25	6	$ (47)	$ 49	669	33	147
Charges associated with litigation matters	(16)	35	0	0					19		25
Selling, general and administrative expense, provisional										(36)
Selling, general and administrative expense, measurement period adjustment									1
Tax expense, provisional										232	0
Tax expense, measurement period adjustment									(3)
Total charges (benefits) associated with U.S. tax reform	3	(5)	0	0	196	0	0	0	(2)	196
Debt extinguishment costs					0	209	0	0		209
Long-term care guaranty fund assessment					0	0	0	83		$ 83
Risk corridor allowance					0	0	0	0			$ 80
Total impact of special items, after-tax	$ 389	$ 138	$ 109	$ 50	$ 221	$ 215	$ (47)	$ 132
Selling, general and administrative expenses [Member]
Before-tax [Abstract]
Transaction-related costs									748
After-tax [Abstract]
Transaction-related costs									587
Interest expense and other [Member]
Before-tax [Abstract]
Transaction-related costs									227
After-tax [Abstract]
Transaction-related costs									179
Net Investment Income [Member]
Before-tax [Abstract]
Transaction-related costs									(123)
After-tax [Abstract]
Transaction-related costs									$ (97)