Fair Value Measurements - Separate Account Assets (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		$ 508		$ 500
Non-guaranteed separate accounts	[1]	6,841		6,584
Subtotal		7,349		7,084
Non-guaranteed separate accounts priced at NAV as a practical expedient	[1]	848		856
Total separate account assets		8,197		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		3,800		3,700
Fair Value Inputs Level 1 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		245		238
Non-guaranteed separate accounts		1,435		1,368
Subtotal		1,680		1,606
Fair Value Inputs Level 2 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		263		262
Non-guaranteed separate accounts		5,076		4,885
Subtotal		5,339		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		330		331
Subtotal		330		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)		27	(1)
Purchases, sales, and settlements:
Purchases		10	4
Sales		(35)	(1)
Settlements		(1)	(2)
Total purchases, sales, and settlements		(26)	1
Transfers into/(out of) Level 3:
Transfers into Level 3		0	23
Transfers out of Level 3		(2)	(3)
Total transfers into/(out of) Level 3		(2)	20
Ending Balance		$ 330	$ 317

[1] Non-guaranteed separate accounts included $3.8 billion as of March 31, 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 March 31, 2017 and $0.9 billion as of December 31, 2016 priced at net asset value ("NAV") as a practical expedient.