Investments - Gross Unrealized Appreciation (Depreciation) on Fixed Maturities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 23,123	$ 21,867
Unrealized Appreciation	890	1,371
Unrealized (Depreciation)	(420)	(100)
Total Fair Value	23,593	23,138
Run-off Settlement Annuity Business [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,282	2,200
Unrealized Appreciation	525	681
Unrealized (Depreciation)	(25)	(2)
Total Fair Value	2,782	2,879
Federal government and agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	575	541
Unrealized Appreciation	209	239
Unrealized (Depreciation)	(4)	(1)
Total Fair Value	780	779
State and local government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	978	1,196
Unrealized Appreciation	69	93
Unrealized (Depreciation)	(2)	(2)
Total Fair Value	1,045	1,287
Foreign government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,354	2,360
Unrealized Appreciation	120	142
Unrealized (Depreciation)	(19)	(15)
Total Fair Value	2,455	2,487
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,697	17,301
Unrealized Appreciation	473	868
Unrealized (Depreciation)	(385)	(81)
Total Fair Value	18,785	18,088
Mortgage and other asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	519	469
Unrealized Appreciation	19	29
Unrealized (Depreciation)	(10)	(1)
Total Fair Value	$ 528	$ 497