Fair Value Measurements - Separate Account Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	$ 523	$ 500
Non-guaranteed separate accounts	7,126	6,584
Subtotal	7,649	7,084
Non-guaranteed separate accounts priced at NAV as a practical expedient	774	856
Total separate account assets	8,423	7,940
Pension Benefits [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Non-guaranteed separate accounts priced at NAV as a practical expedient	800	900
Non-guaranteed separate accounts	3,900	3,700
Fair Value Inputs Level 1 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	215	238
Non-guaranteed separate accounts	1,536	1,368
Subtotal	1,751	1,606
Fair Value Inputs Level 2 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	308	262
Non-guaranteed separate accounts	5,298	4,885
Subtotal	5,606	5,147
Fair Value Inputs Level 3 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	0	0
Non-guaranteed separate accounts	292	331
Subtotal	292	331
Fair Value Inputs Level 3 [Member] | Pension Benefits [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Non-guaranteed separate accounts	300	300
Separate Account Assets [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	331	297
Policyholder gains (losses)	34	2
Purchases, sales, and settlements:
Purchases	33	22
Sales	(53)	(11)
Settlements	(13)	(18)
Total purchases, sales, and settlements	(33)	(7)
Transfers into/(out of) Level 3:
Transfers into Level 3	7	65
Transfers out of Level 3	(47)	(26)
Total transfers into/(out of) Level 3	(40)	39
Ending Balance	$ 292	$ 331