Investments - Total unrealized losses (Details) $ in Millions	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Fixed income and equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	670	888
Fair value, continuous unrealized loss position for less than 12 months	$ 3,596	$ 5,270
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (34)	$ (145)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	218	188
Fair value, continuous unrealized loss position for 12 months or more	$ 1,425	$ 641
Unrealized losses, continuous unrealized loss position for 12 months or more	(67)	(90)
Total unrealized losses
Total unrealized losses	$ (101)	$ (235)
Fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	583	721
Fair value, continuous unrealized loss position for less than 12 months	$ 3,530	$ 5,070
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (31)	$ (126)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	217	126
Fair value, continuous unrealized loss position for 12 months or more	$ 1,425	$ 561
Unrealized losses, continuous unrealized loss position for 12 months or more	(67)	(74)
Total unrealized losses
Gross unrealized losses	$ (98)	$ (200)
U.S. government and agencies
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	17	6
Fair value, continuous unrealized loss position for less than 12 months	$ 443	$ 104
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (2)	$ (2)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	2	0
Fair value, continuous unrealized loss position for 12 months or more	$ 25	$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more	0	0
Total unrealized losses
Total unrealized losses	$ (2)	$ (2)
Municipal
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	4	8
Fair value, continuous unrealized loss position for less than 12 months	$ 14	$ 44
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0	$ (1)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	1	3
Fair value, continuous unrealized loss position for 12 months or more	$ 11	$ 18
Unrealized losses, continuous unrealized loss position for 12 months or more	(3)	(6)
Total unrealized losses
Total unrealized losses	$ (3)	$ (7)
Corporate
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	456	629
Fair value, continuous unrealized loss position for less than 12 months	$ 2,899	$ 4,767
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (28)	$ (118)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	144	56
Fair value, continuous unrealized loss position for 12 months or more	$ 1,324	$ 414
Unrealized losses, continuous unrealized loss position for 12 months or more	(58)	(51)
Total unrealized losses
Total unrealized losses	$ (86)	$ (169)
ABS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	33	18
Fair value, continuous unrealized loss position for less than 12 months	$ 170	$ 95
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (1)	$ (1)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	8	13
Fair value, continuous unrealized loss position for 12 months or more	$ 24	$ 76
Unrealized losses, continuous unrealized loss position for 12 months or more	(3)	(9)
Total unrealized losses
Total unrealized losses	$ (4)	$ (10)
RMBS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	70	47
Fair value, continuous unrealized loss position for less than 12 months	$ 3	$ 3
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0	$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	56	50
Fair value, continuous unrealized loss position for 12 months or more	$ 18	$ 38
Unrealized losses, continuous unrealized loss position for 12 months or more	(1)	(3)
Total unrealized losses
Total unrealized losses	$ (1)	$ (3)
CMBS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	2	12
Fair value, continuous unrealized loss position for less than 12 months	$ 1	$ 57
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0	$ (4)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	6	4
Fair value, continuous unrealized loss position for 12 months or more	$ 23	$ 15
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(5)
Total unrealized losses
Total unrealized losses	$ (2)	$ (9)
Redeemable preferred stock
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	1	1
Fair value, continuous unrealized loss position for less than 12 months	$ 0	$ 0
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0	$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	0	0
Fair value, continuous unrealized loss position for 12 months or more	$ 0	$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more	0	0
Total unrealized losses
Total unrealized losses	$ 0	$ 0
Investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	472	559
Fair value, continuous unrealized loss position for less than 12 months	$ 3,192	$ 4,348
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (22)	$ (100)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	181	75
Fair value, continuous unrealized loss position for 12 months or more	$ 1,320	$ 350
Unrealized losses, continuous unrealized loss position for 12 months or more	(52)	(47)
Total unrealized losses
Total unrealized losses	$ (74)	$ (147)
Below investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	111	162
Fair value, continuous unrealized loss position for less than 12 months	$ 338	$ 722
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (9)	$ (26)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	36	51
Fair value, continuous unrealized loss position for 12 months or more	$ 105	$ 211
Unrealized losses, continuous unrealized loss position for 12 months or more	(15)	(27)
Total unrealized losses
Total unrealized losses	$ (24)	$ (53)
Equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	87	167
Fair value, continuous unrealized loss position for less than 12 months	$ 66	$ 200
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (3)	$ (19)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | security	1	62
Fair value, continuous unrealized loss position for 12 months or more	$ 0	$ 80
Unrealized losses, continuous unrealized loss position for 12 months or more	0	(16)
Total unrealized losses
Gross unrealized losses on equity securities	$ (3)	$ (35)