Investments - Gross Unrealized Appreciation (Depreciation) on Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 21,867	$ 19,942
Unrealized Appreciation	1,371	1,196
Unrealized (Depreciation)	(100)	(177)
Total Fair Value	23,138	20,961
Run-off Settlement Annuity Business [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,200	2,196
Unrealized Appreciation	681	539
Unrealized (Depreciation)	(2)	(15)
Total Fair Value	2,879	2,720
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Commitments to make additional investments	118
Federal government and agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	541	658
Unrealized Appreciation	239	223
Unrealized (Depreciation)	(1)	(4)
Total Fair Value	779	877
State and local government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,196	1,342
Unrealized Appreciation	93	99
Unrealized (Depreciation)	(2)	(6)
Total Fair Value	1,287	1,435
Foreign government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,360	1,998
Unrealized Appreciation	142	129
Unrealized (Depreciation)	(15)	(14)
Total Fair Value	2,487	2,113
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,301	15,483
Unrealized Appreciation	868	716
Unrealized (Depreciation)	(81)	(149)
Total Fair Value	18,088	16,050
Mortgage and other asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	469	461
Unrealized Appreciation	29	29
Unrealized (Depreciation)	(1)	(4)
Total Fair Value	$ 497	$ 486