Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Cost or Amortized Cost	$ 38,874	$ 38,467
Gross Unrealized Gains	3,397	2,744
Gross Unrealized Losses	89	196
Estimated Fair Value	42,182	41,015
Corporate and other bonds
Cost or Amortized Cost	17,210	17,711
Gross Unrealized Gains	1,625	1,323
Gross Unrealized Losses	28	76
Estimated Fair Value	18,807	18,958
Unrealized OTTI Losses (Gains)	0	(1)
States, municipalities and political subdivisions
Cost or Amortized Cost	12,478	12,060
Gross Unrealized Gains	1,551	1,213
Gross Unrealized Losses	2	33
Estimated Fair Value	14,027	13,240
Unrealized OTTI Losses (Gains)	(11)	(16)
Residential mortgage-backed
Cost or Amortized Cost	5,043	5,004
Gross Unrealized Gains	109	120
Gross Unrealized Losses	32	51
Estimated Fair Value	5,120	5,073
Unrealized OTTI Losses (Gains)	(27)	(28)
Commercial mortgage-backed
Cost or Amortized Cost	1,840	2,016
Gross Unrealized Gains	46	48
Gross Unrealized Losses	14	24
Estimated Fair Value	1,872	2,040
Unrealized OTTI Losses (Gains)	0	0
Other asset-backed
Cost or Amortized Cost	1,083	1,022
Gross Unrealized Gains	16	8
Gross Unrealized Losses	5	5
Estimated Fair Value	1,094	1,025
Unrealized OTTI Losses (Gains)	0	0
Total asset-backed
Cost or Amortized Cost	7,966	8,042
Gross Unrealized Gains	171	176
Gross Unrealized Losses	51	80
Estimated Fair Value	8,086	8,138
Unrealized OTTI Losses (Gains)	(27)	(28)
U.S. Treasury and obligations of government-sponsored enterprises
Cost or Amortized Cost	111	83
Gross Unrealized Gains	2	10
Gross Unrealized Losses	4	0
Estimated Fair Value	109	93
Unrealized OTTI Losses (Gains)	0	0
Foreign government
Cost or Amortized Cost	437	435
Gross Unrealized Gains	9	13
Gross Unrealized Losses	2	3
Estimated Fair Value	444	445
Unrealized OTTI Losses (Gains)	0	0
Redeemable preferred stock
Cost or Amortized Cost	10	18
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value	11	19
Unrealized OTTI Losses (Gains)	0	0
Fixed maturity securities
Cost or Amortized Cost	38,212	38,349
Gross Unrealized Gains	3,359	2,736
Gross Unrealized Losses	87	192
Estimated Fair Value	41,484	40,893
Unrealized OTTI Losses (Gains)	(38)	(45)
Total fixed maturity securities trading
Cost or amortized cost, trading securities	3	12
Estimated fair value, trading securities	3	12
Common Stock
Cost or Amortized Cost	21	13
Gross Unrealized Gains	7	6
Gross Unrealized Losses	1	0
Estimated Fair Value	27	19
Preferred stock
Cost or Amortized Cost	638	93
Gross Unrealized Gains	31	2
Gross Unrealized Losses	1	4
Estimated Fair Value	668	91
Equity securities
Cost or Amortized Cost	659	106
Gross Unrealized Gains	38	8
Gross Unrealized Losses	2	4
Estimated Fair Value	$ 695	$ 110