Investments - Gross Unrealized Appreciation (Depreciation) on Fixed Maturities (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 21,670,000,000	$ 19,942,000,000
Unrealized Appreciation	1,369,000,000	1,196,000,000
Unrealized (Depreciation)	(95,000,000)	(177,000,000)
Total Fair Value	22,944,000,000	20,961,000,000
Run-off Settlement Annuity Business [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,172,000,000	2,196,000,000
Unrealized Appreciation	630,000,000	539,000,000
Unrealized (Depreciation)	(3,000,000)	(15,000,000)
Total Fair Value	2,799,000,000	2,720,000,000
Federal government and agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	666,000,000	658,000,000
Unrealized Appreciation	238,000,000	223,000,000
Unrealized (Depreciation)	(2,000,000)	(4,000,000)
Total Fair Value	902,000,000	877,000,000
State and local government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,223,000,000	1,342,000,000
Unrealized Appreciation	102,000,000	99,000,000
Unrealized (Depreciation)	(2,000,000)	(6,000,000)
Total Fair Value	1,323,000,000	1,435,000,000
Foreign government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,205,000,000	1,998,000,000
Unrealized Appreciation	119,000,000	129,000,000
Unrealized (Depreciation)	(17,000,000)	(14,000,000)
Total Fair Value	2,307,000,000	2,113,000,000
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,130,000,000	15,483,000,000
Unrealized Appreciation	876,000,000	716,000,000
Unrealized (Depreciation)	(73,000,000)	(149,000,000)
Total Fair Value	17,933,000,000	16,050,000,000
Mortgage and other asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	446,000,000	461,000,000
Unrealized Appreciation	34,000,000	29,000,000
Unrealized (Depreciation)	(1,000,000)	(4,000,000)
Total Fair Value	$ 479,000,000	$ 486,000,000