Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Cost or Amortized Cost	$ 37,991	$ 38,874
Gross Unrealized Gains	2,514	3,397
Gross Unrealized Losses	246	89
Estimated Fair Value	40,259	42,182
Corporate and other bonds
Cost or Amortized Cost	17,538	17,210
Gross Unrealized Gains	1,177	1,625
Gross Unrealized Losses	115	28
Estimated Fair Value	18,600	18,807
Unrealized OTTI Losses (Gains)	0	0
States, municipalities and political subdivisions
Cost or Amortized Cost	11,682	12,478
Gross Unrealized Gains	1,205	1,551
Gross Unrealized Losses	12	2
Estimated Fair Value	12,875	14,027
Unrealized OTTI Losses (Gains)	0	(11)
Residential mortgage-backed
Cost or Amortized Cost	5,050	5,043
Gross Unrealized Gains	83	109
Gross Unrealized Losses	78	32
Estimated Fair Value	5,055	5,120
Unrealized OTTI Losses (Gains)	(27)	(27)
Commercial mortgage-backed
Cost or Amortized Cost	1,948	1,840
Gross Unrealized Gains	29	46
Gross Unrealized Losses	22	14
Estimated Fair Value	1,955	1,872
Unrealized OTTI Losses (Gains)	0	0
Other asset-backed
Cost or Amortized Cost	1,185	1,083
Gross Unrealized Gains	10	16
Gross Unrealized Losses	7	5
Estimated Fair Value	1,188	1,094
Unrealized OTTI Losses (Gains)	0	0
Total asset-backed
Cost or Amortized Cost	8,183	7,966
Gross Unrealized Gains	122	171
Gross Unrealized Losses	107	51
Estimated Fair Value	8,198	8,086
Unrealized OTTI Losses (Gains)	(27)	(27)
U.S. Treasury and obligations of government-sponsored enterprises
Cost or Amortized Cost	124	111
Gross Unrealized Gains	2	2
Gross Unrealized Losses	7	4
Estimated Fair Value	119	109
Unrealized OTTI Losses (Gains)	0	0
Foreign government
Cost or Amortized Cost	448	437
Gross Unrealized Gains	7	9
Gross Unrealized Losses	5	2
Estimated Fair Value	450	444
Unrealized OTTI Losses (Gains)	0	0
Redeemable preferred stock
Cost or Amortized Cost	9	10
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value	10	11
Unrealized OTTI Losses (Gains)	0	0
Fixed maturity securities
Cost or Amortized Cost	37,984	38,212
Gross Unrealized Gains	2,514	3,359
Gross Unrealized Losses	246	87
Estimated Fair Value	40,252	41,484
Unrealized OTTI Losses (Gains)	(27)	(38)
Total fixed maturity securities trading
Cost or Amortized Cost	7	3
Estimated Fair Value	$ 7	3
Common Stock
Cost or Amortized Cost		21
Gross Unrealized Gains		7
Gross Unrealized Losses		1
Estimated Fair Value		27
Preferred stock
Cost or Amortized Cost		638
Gross Unrealized Gains		31
Gross Unrealized Losses		1
Estimated Fair Value		668
Equity securities
Cost or Amortized Cost		659
Gross Unrealized Gains		38
Gross Unrealized Losses		2
Estimated Fair Value		$ 695