Investments - Gross Unrealized Appreciation (Depreciation) on Fixed Maturities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 23,416	$ 21,867
Unrealized Appreciation	1,043	1,371
Unrealized (Depreciation)	(281)	(100)
Total Fair Value	24,178	23,138
Run-off Settlement Annuity Business [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,241	2,200
Unrealized Appreciation	576	681
Unrealized (Depreciation)	(9)	(2)
Total Fair Value	2,808	2,879
Federal government and agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	653	541
Unrealized Appreciation	219	239
Unrealized (Depreciation)	(4)	(1)
Total Fair Value	868	779
State and local government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,058	1,196
Unrealized Appreciation	72	93
Unrealized (Depreciation)	(1)	(2)
Total Fair Value	1,129	1,287
Foreign government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,402	2,360
Unrealized Appreciation	119	142
Unrealized (Depreciation)	(22)	(15)
Total Fair Value	2,499	2,487
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,812	17,301
Unrealized Appreciation	610	868
Unrealized (Depreciation)	(249)	(81)
Total Fair Value	19,173	18,088
Mortgage and other asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	491	469
Unrealized Appreciation	23	29
Unrealized (Depreciation)	(5)	(1)
Total Fair Value	$ 509	$ 497