Fair Value Measurements - Separate Account Assets (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		$ 518		$ 518		$ 500
Non-guaranteed separate accounts	[1]	7,117		7,117		6,584
Subtotal		7,635		7,635		7,084
Non-guaranteed separate accounts priced at NAV as a practical expedient	[1]	833		833		856
Total separate account assets		8,468		8,468		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		800		900
Non-guaranteed separate accounts		3,900		3,900		3,700
Fair Value Inputs Level 1 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		229		229		238
Non-guaranteed separate accounts		1,438		1,438		1,368
Subtotal		1,667		1,667		1,606
Fair Value Inputs Level 2 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		289		289		262
Non-guaranteed separate accounts		5,363		5,363		4,885
Subtotal		5,652		5,652		5,147
Fair Value Inputs Level 3 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		0		0		0
Non-guaranteed separate accounts		316		316		331
Subtotal		316		316		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		$ 300
Separate Account Assets [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		330	$ 317	331	$ 297
Policyholder gains (losses)		10	2	37	1
Purchases, sales, and settlements:
Purchases		7	1	17	5
Sales		(17)	0	(52)	(1)
Settlements		0	0	(1)	(2)
Total purchases, sales, and settlements		(10)	1	(36)	2
Transfers into/(out of) Level 3:
Transfers into Level 3		1	16	1	39
Transfers out of Level 3		(15)	(3)	(17)	(6)
Total transfers into/(out of) Level 3		(14)	13	(16)	33
Ending Balance		$ 316	$ 333	$ 316	$ 333

[1] Non-guaranteed separate accounts included $3.9 billion as of June 30, 2017 and $3.7 billion as of December 31, 2016 in assets supporting the Company's pension plans, including $0.3 billion classified in Level 3 for both periods and $0.8 billion as of June 30, 2017 and $0.9 billion as of December 31, 2016 priced at net asset value ("NAV") as a practical expedient.