Fair Value Measurements - Separate Account Assets (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		$ 529		$ 529		$ 500
Non-guaranteed separate accounts	[1]	7,051		7,051		6,584
Subtotal		7,580		7,580		7,084
Non-guaranteed separate accounts priced at NAV as a practical expedient	[1]	852		852		856
Total separate account assets		8,432		8,432		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		216		216		238
Non-guaranteed separate accounts		1,511		1,511		1,368
Subtotal		1,727		1,727		1,606
Fair Value Inputs Level 2 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		313		313		262
Non-guaranteed separate accounts		5,240		5,240		4,885
Subtotal		5,553		5,553		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		300		300		331
Subtotal		300		300		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		316	$ 333	331	$ 297
Policyholder gains (losses)		(1)	2	36	3
Purchases, sales, and settlements:
Purchases		9	7	26	12
Sales		0	0	(52)	(1)
Settlements		(12)	(3)	(13)	(5)
Total purchases, sales, and settlements		(3)	4	(39)	6
Transfers into/(out of) Level 3:
Transfers into Level 3		4	7	5	46
Transfers out of Level 3		(16)	0	(33)	(6)
Total transfers into/(out of) Level 3		(12)	7	(28)	40
Ending Balance		$ 300	$ 346	$ 300	$ 346

[1] Non-guaranteed separate accounts included $3.9 billion as of September 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 September 30, 2017 and $0.9 billion as of December 31, 2016 priced at net asset value ("NAV") as a practical expedient.