Investments - Gross unrealized losses and fair value by the type and length of time held in continuous unrealized loss position (Details) $ in Millions	Mar. 31, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract
Fixed income and equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract		2,953
Fair value, continuous unrealized loss position for less than 12 months		$ 19,049
Unrealized losses, continuous unrealized loss position for less than 12 months		$ (170)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract		594
Fair value, continuous unrealized loss position for 12 months or more		$ 3,971
Unrealized losses, continuous unrealized loss position for 12 months or more		(125)
Total unrealized losses
Total unrealized losses	$ (745)	$ (295)
U.S. government and agencies
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	57	66
Fair value, continuous unrealized loss position for less than 12 months	$ 1,514	$ 2,829
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (16)	$ (18)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	13	18
Fair value, continuous unrealized loss position for 12 months or more	$ 74	$ 182
Unrealized losses, continuous unrealized loss position for 12 months or more	(1)	(2)
Total unrealized losses
Total unrealized losses	$ (17)	$ (20)
Municipal
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	2,535	1,756
Fair value, continuous unrealized loss position for less than 12 months	$ 4,626	$ 3,143
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (74)	$ (24)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	160	165
Fair value, continuous unrealized loss position for 12 months or more	$ 323	$ 349
Unrealized losses, continuous unrealized loss position for 12 months or more	(18)	(12)
Total unrealized losses
Total unrealized losses	$ (92)	$ (36)
Corporate
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	1,656	781
Fair value, continuous unrealized loss position for less than 12 months	$ 22,720	$ 11,616
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (458)	$ (102)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	199	208
Fair value, continuous unrealized loss position for 12 months or more	$ 3,006	$ 3,289
Unrealized losses, continuous unrealized loss position for 12 months or more	(153)	(102)
Total unrealized losses
Total unrealized losses	$ (611)	$ (204)
Foreign government
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	43	45
Fair value, continuous unrealized loss position for less than 12 months	$ 531	$ 580
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (9)	$ (10)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	4	5
Fair value, continuous unrealized loss position for 12 months or more	$ 39	$ 44
Unrealized losses, continuous unrealized loss position for 12 months or more	(1)	(1)
Total unrealized losses
Total unrealized losses	$ (10)	$ (11)
Asset-backed securities (“ABS”)
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	80	57
Fair value, continuous unrealized loss position for less than 12 months	$ 541	$ 476
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (6)	$ (3)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	7	9
Fair value, continuous unrealized loss position for 12 months or more	$ 11	$ 34
Unrealized losses, continuous unrealized loss position for 12 months or more	(4)	(4)
Total unrealized losses
Total unrealized losses	$ (10)	$ (7)
Residential mortgage-backed securities (“RMBS”)
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	139	118
Fair value, continuous unrealized loss position for less than 12 months	$ 36	$ 35
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 | contract	181	181
Fair value, continuous unrealized loss position for 12 months or more	$ 43	$ 50
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(2)
Total unrealized losses
Total unrealized losses	$ (3)	$ (3)
Commercial mortgage-backed securities (“CMBS”)
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	0	2
Fair value, continuous unrealized loss position for less than 12 months	$ 0	$ 1
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 | contract	6	6
Fair value, continuous unrealized loss position for 12 months or more	$ 24	$ 23
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(2)
Total unrealized losses
Total unrealized losses	$ (2)	$ (2)
Redeemable preferred stock
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract		1
Fair value, continuous unrealized loss position for less than 12 months		$ 0
Unrealized losses, continuous unrealized loss position for less than 12 months		$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract		0
Fair value, continuous unrealized loss position for 12 months or more		$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more		0
Total unrealized losses
Total unrealized losses		$ 0
Fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	4,510	2,826
Fair value, continuous unrealized loss position for less than 12 months	$ 29,968	$ 18,680
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (564)	$ (158)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	570	592
Fair value, continuous unrealized loss position for 12 months or more	$ 3,520	$ 3,971
Unrealized losses, continuous unrealized loss position for 12 months or more	(181)	(125)
Total unrealized losses
Total unrealized losses	$ (745)	$ (283)
Equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract		127
Fair value, continuous unrealized loss position for less than 12 months		$ 369
Unrealized losses, continuous unrealized loss position for less than 12 months		$ (12)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract		2
Fair value, continuous unrealized loss position for 12 months or more		$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more		0
Total unrealized losses
Total unrealized losses		$ (12)
Investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	4,161	2,706
Fair value, continuous unrealized loss position for less than 12 months	$ 27,020	$ 17,668
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (470)	$ (134)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	516	535
Fair value, continuous unrealized loss position for 12 months or more	$ 3,284	$ 3,751
Unrealized losses, continuous unrealized loss position for 12 months or more	(153)	(98)
Total unrealized losses
Total unrealized losses	$ (623)	$ (232)
Below investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	349	120
Fair value, continuous unrealized loss position for less than 12 months	$ 2,948	$ 1,012
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (94)	$ (24)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	54	57
Fair value, continuous unrealized loss position for 12 months or more	$ 236	$ 220
Unrealized losses, continuous unrealized loss position for 12 months or more	(28)	(27)
Total unrealized losses
Total unrealized losses	$ (122)	$ (51)