Segment Information - Special Items (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Apr. 30, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
After-tax [Abstract]
Other operating expenses (see Note 20 for details)										$ (36)
Tax expense (see Note 20 for details)										232	$ 0	$ 0
Total charges associated with U.S. tax reform		$ 196	$ 0	$ 0	$ 0					196
Debt extinguishment costs	$ 65	0	209	0	0					209		65
Long-term care guaranty fund assessment		0	0	0	83					83
Transaction-related costs		$ 25	6	$ (47)	$ 49	$ 39	$ 46	$ 26	$ 36	33	147	57
Risk corridor allowance						80	0	0	0		80
Charges associated with litigation matters						$ 0	$ 25	$ 0	$ 0		25
Before-tax [Abstract]
Other operating expenses (see note 20 for additional details)										(56)
Total charges associated with U.S. tax reform										(56)
Debt extinguishment costs	$ 100		$ 321							321	0	100
Long-term care guaranty fund assessment										129
Transaction-related costs										126	166	66
Risk corridor allowance											124
Charges associated with litigation matters											40
Other Operating Expenses [Member]
After-tax [Abstract]
Other operating expenses (see Note 20 for details)										(36)
Debt extinguishment costs										209		65
Long-term care guaranty fund assessment										83
Transaction-related costs										33	147	57
Risk corridor allowance											80
Charges associated with litigation matters											25
Before-tax [Abstract]
Other operating expenses (see note 20 for additional details)										(56)
Debt extinguishment costs										321		100
Long-term care guaranty fund assessment										129
Transaction-related costs										$ 126	166	$ 66
Risk corridor allowance											124
Charges associated with litigation matters											$ 40