Fair Value Measurements - Separate Account Assets (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		$ 517		$ 517		$ 509
Non-guaranteed separate accounts	[1]	6,700		6,700		6,396
Subtotal		7,217		7,217		6,905
Non-guaranteed separate accounts priced at NAV as a practical expedient	[1]	939		939		928
Total separate account assets		8,156		8,156		7,833
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		900		900		900
Non-guaranteed separate accounts		3,700		3,700		3,600
Fair Value Inputs Level 1 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		246		246		235
Non-guaranteed separate accounts		1,407		1,407		1,401
Subtotal		1,653		1,653		1,636
Fair Value Inputs Level 2 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts		271		271		274
Non-guaranteed separate accounts		4,947		4,947		4,698
Subtotal		5,218		5,218		4,972
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		346		346		297
Subtotal		346		346		297
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		333	$ 273	297	$ 255
Policyholder gains (losses)		2	(1)	3	(3)
Purchases, sales, and settlements:
Purchases		7	4	12	33
Sales		0	0	(1)	0
Settlements		(3)	(3)	(5)	(8)
Total purchases, sales, and settlements		4	1	6	25
Transfers into/(out of) Level 3:
Transfers into Level 3		7	16	46	16
Transfers out of Level 3		0	(3)	(6)	(7)
Total transfers into/(out of) Level 3		7	13	40	9
Ending Balance		$ 346	$ 286	$ 346	$ 286

[1] Non-guaranteed separate accounts included $3.7 billion as of September 30, 2016 and $3.6 billion as of December 31, 2015 in assets supporting the Company's pension plans, including $0.3 billion classified in Level 3 and $0.9 billion priced at NAV as a practical expedient for both periods.