Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 6,989	$ 7,464
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	167	342
Estimated Fair Value, 12 Months or Longer	584	502
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	29	50
Estimated Fair Value, Total	7,573	7,966
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	196	392
Corporate and other bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	2,615	4,882
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	61	302
Estimated Fair Value, 12 Months or Longer	254	162
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	15	40
Estimated Fair Value, Total	2,869	5,044
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	76	342
States, municipalities and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	959	338
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	32	8
Estimated Fair Value, 12 Months or Longer	23	75
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	0
Estimated Fair Value, Total	982	413
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	33	8
Residential mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	2,136	963
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	44	9
Estimated Fair Value, 12 Months or Longer	201	164
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7	8
Estimated Fair Value, Total	2,337	1,127
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	51	17
Commercial mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	756	652
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	22	10
Estimated Fair Value, 12 Months or Longer	69	96
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	2
Estimated Fair Value, Total	825	748
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	24	12
Other asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	398	552
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5	8
Estimated Fair Value, 12 Months or Longer	24	5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value, Total	422	557
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5	8
Total asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	3,290	2,167
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	71	27
Estimated Fair Value, 12 Months or Longer	294	265
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	9	10
Estimated Fair Value, Total	3,584	2,432
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	80	37
U.S. Treasury and obligations of government-sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	5	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Estimated Fair Value, 12 Months or Longer	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value, Total	5	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	0
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	108	54
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3	1
Estimated Fair Value, 12 Months or Longer	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value, Total	108	54
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	3	1
Redeemable preferred stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	0	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Estimated Fair Value, 12 Months or Longer	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value, Total	0	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	0
Total fixed maturity securities available-for-sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	6,977	7,448
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	167	338
Estimated Fair Value, 12 Months or Longer	571	502
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	25	50
Estimated Fair Value, Total	7,548	7,950
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	192	388
Common stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months		3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		1
Estimated Fair Value, 12 Months or Longer		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Estimated Fair Value, Total		3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		1
Preferred stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	12	13
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	3
Estimated Fair Value, 12 Months or Longer	13	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4	0
Estimated Fair Value, Total	25	13
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 4	3
Total equity securities available-for-sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months		16
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		4
Estimated Fair Value, 12 Months or Longer		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Estimated Fair Value, Total		16
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ 4