Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Cost or Amortized Cost	$ 37,911	$ 38,212
Gross Unrealized Gains	2,213
Gross Unrealized Losses	346
Estimated Fair Value	39,778	41,484
Cost or Amortized Cost	37,928
Estimated Fair Value	39,795
Cost or Amortized Cost		38,874
Gross Unrealized Gains		3,397
Gross Unrealized Losses		89
Estimated Fair Value		42,182
Corporate and other bonds
Cost or Amortized Cost	17,883
Gross Unrealized Gains	938
Gross Unrealized Losses	203
Estimated Fair Value	18,618
Cost or Amortized Cost		17,210
Gross Unrealized Gains		1,625
Gross Unrealized Losses		28
Estimated Fair Value		18,807
Unrealized OTTI Losses (Gains)	0	0
States, municipalities and political subdivisions
Cost or Amortized Cost	10,856
Gross Unrealized Gains	1,161
Gross Unrealized Losses	8
Estimated Fair Value	12,009
Cost or Amortized Cost		12,478
Gross Unrealized Gains		1,551
Gross Unrealized Losses		2
Estimated Fair Value		14,027
Unrealized OTTI Losses (Gains)	0	(11)
Residential mortgage-backed
Cost or Amortized Cost	4,961
Gross Unrealized Gains	72
Gross Unrealized Losses	85
Estimated Fair Value	4,948
Cost or Amortized Cost		5,043
Gross Unrealized Gains		109
Gross Unrealized Losses		32
Estimated Fair Value		5,120
Unrealized OTTI Losses (Gains)	(25)	(27)
Commercial mortgage-backed
Cost or Amortized Cost	2,082
Gross Unrealized Gains	23
Gross Unrealized Losses	33
Estimated Fair Value	2,072
Cost or Amortized Cost		1,840
Gross Unrealized Gains		46
Gross Unrealized Losses		14
Estimated Fair Value		1,872
Unrealized OTTI Losses (Gains)	0	0
Other asset-backed
Cost or Amortized Cost	1,557
Gross Unrealized Gains	9
Gross Unrealized Losses	9
Estimated Fair Value	1,557
Cost or Amortized Cost		1,083
Gross Unrealized Gains		16
Gross Unrealized Losses		5
Estimated Fair Value		1,094
Unrealized OTTI Losses (Gains)	0	0
Total asset-backed
Cost or Amortized Cost	8,600
Gross Unrealized Gains	104
Gross Unrealized Losses	127
Estimated Fair Value	8,577
Cost or Amortized Cost		7,966
Gross Unrealized Gains		171
Gross Unrealized Losses		51
Estimated Fair Value		8,086
Unrealized OTTI Losses (Gains)	(25)	(27)
U.S. Treasury and obligations of government-sponsored enterprises
Cost or Amortized Cost	127
Gross Unrealized Gains	3
Gross Unrealized Losses	3
Estimated Fair Value	127
Cost or Amortized Cost		111
Gross Unrealized Gains		2
Gross Unrealized Losses		4
Estimated Fair Value		109
Unrealized OTTI Losses (Gains)	0	0
Foreign government
Cost or Amortized Cost	436
Gross Unrealized Gains	6
Gross Unrealized Losses	5
Estimated Fair Value	437
Cost or Amortized Cost		437
Gross Unrealized Gains		9
Gross Unrealized Losses		2
Estimated Fair Value		444
Unrealized OTTI Losses (Gains)	0	0
Redeemable preferred stock
Cost or Amortized Cost	9
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Estimated Fair Value	10
Cost or Amortized Cost		10
Gross Unrealized Gains		1
Gross Unrealized Losses		0
Estimated Fair Value		11
Unrealized OTTI Losses (Gains)	0	0
Fixed maturity securities
Cost or Amortized Cost	37,911
Gross Unrealized Gains	2,213
Gross Unrealized Losses	346
Estimated Fair Value	39,778
Cost or Amortized Cost		38,212
Gross Unrealized Gains		3,359
Gross Unrealized Losses		87
Estimated Fair Value		41,484
Unrealized OTTI Losses (Gains)	(25)	(38)
Total fixed maturity securities trading
Cost or Amortized Cost	17
Estimated Fair Value	$ 17
Cost or Amortized Cost		3
Estimated Fair Value		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