Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 3,620	$ 6,989
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	40	167
Estimated Fair Value, 12 Months or Longer	1,455	584
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	49	29
Estimated Fair Value, Total	5,075	7,573
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	89	196
Corporate and other bonds
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,354	2,615
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	21	61
Estimated Fair Value, 12 Months or Longer	168	254
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7	15
Estimated Fair Value, Total	1,522	2,869
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	28	76
States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	72	959
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	32
Estimated Fair Value, 12 Months or Longer	85	23
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	1
Estimated Fair Value, Total	157	982
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	33
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,228	2,136
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5	44
Estimated Fair Value, 12 Months or Longer	947	201
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	27	7
Estimated Fair Value, Total	2,175	2,337
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	32	51
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	403	756
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	22
Estimated Fair Value, 12 Months or Longer	212	69
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	10	2
Estimated Fair Value, Total	615	825
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	14	24
Other asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	248	398
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3	5
Estimated Fair Value, 12 Months or Longer	18	24
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	0
Estimated Fair Value, Total	266	422
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5	5
Total asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,879	3,290
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	12	71
Estimated Fair Value, 12 Months or Longer	1,177	294
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	39	9
Estimated Fair Value, Total	3,056	3,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	51	80
U.S. Treasury and obligations of government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	49	5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	0
Estimated Fair Value, 12 Months or Longer	21	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	0
Estimated Fair Value, Total	70	5
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	4	0
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	166	108
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	3
Estimated Fair Value, 12 Months or Longer	4	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value, Total	170	108
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	3
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	3,520	6,977
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	38	167
Estimated Fair Value, 12 Months or Longer	1,455	571
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	49	25
Estimated Fair Value, Total	4,975	7,548
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	87	192
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	7
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	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	93
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	93
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	100	12
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	0
Estimated Fair Value, 12 Months or Longer	0	13
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	4
Estimated Fair Value, Total	100	25
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 2	$ 4