Investments (Details 11) $ in Millions	Sep. 30, 2017 USD ($) contract	Dec. 31, 2016 USD ($) security contract
Fixed income and equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	1,892	2,590
Fair value, continuous unrealized loss position for less than 12 months	$ 13,511	$ 18,572
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (142)	$ (407)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	391	334
Fair value, continuous unrealized loss position for 12 months or more	$ 1,789	$ 1,087
Unrealized losses, continuous unrealized loss position for 12 months or more	(89)	(125)
Total unrealized losses
Total unrealized losses	$ (231)	$ (532)
U.S. government and agencies
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	64	46
Fair value, continuous unrealized loss position for less than 12 months	$ 2,625	$ 943
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (7)	$ (9)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	7	0
Fair value, continuous unrealized loss position for 12 months or more	$ 93	$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more	(1)	0
Total unrealized losses
Total unrealized losses	$ (8)	$ (9)
Municipal
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	938	1,310
Fair value, continuous unrealized loss position for less than 12 months	$ 1,736	$ 3,073
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (14)	$ (76)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	83	8
Fair value, continuous unrealized loss position for 12 months or more	$ 172	$ 29
Unrealized losses, continuous unrealized loss position for 12 months or more	(8)	(11)
Total unrealized losses
Total unrealized losses	$ (22)	$ (87)
Corporate
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	560	862
Fair value, continuous unrealized loss position for less than 12 months	$ 7,468	$ 13,343
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (74)	$ (256)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	94	83
Fair value, continuous unrealized loss position for 12 months or more	$ 1,370	$ 678
Unrealized losses, continuous unrealized loss position for 12 months or more	(58)	(63)
Total unrealized losses
Total unrealized losses	$ (132)	$ (319)
Foreign government
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	68	41
Fair value, continuous unrealized loss position for less than 12 months	$ 697	$ 225
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (14)	$ (4)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	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	$ (14)	$ (4)
Asset-backed securities (“ABS”)
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	44	31
Fair value, continuous unrealized loss position for less than 12 months	$ 379	$ 222
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	10	14
Fair value, continuous unrealized loss position for 12 months or more	$ 34	$ 109
Unrealized losses, continuous unrealized loss position for 12 months or more	(8)	(10)
Total unrealized losses
Total unrealized losses	$ (9)	$ (11)
Residential mortgage-backed securities (“RMBS”)
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	84	89
Fair value, continuous unrealized loss position for less than 12 months	$ 42	$ 53
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 | contract	176	179
Fair value, continuous unrealized loss position for 12 months or more	$ 57	$ 91
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(7)
Total unrealized losses
Total unrealized losses	$ (2)	$ (8)
Commercial mortgage-backed securities (“CMBS”)
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	5	15
Fair value, continuous unrealized loss position for less than 12 months	$ 20	$ 59
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (2)	$ (4)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	4	4
Fair value, continuous unrealized loss position for 12 months or more	$ 15	$ 15
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(5)
Total unrealized losses
Total unrealized losses	$ (4)	$ (9)
Redeemable preferred stock
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security		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 | security		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	1,763	2,395
Fair value, continuous unrealized loss position for less than 12 months	$ 12,967	$ 17,918
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (112)	$ (351)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	374	288
Fair value, continuous unrealized loss position for 12 months or more	$ 1,741	$ 922
Unrealized losses, continuous unrealized loss position for 12 months or more	(79)	(96)
Total unrealized losses
Total unrealized losses	$ (191)	$ (447)
Equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	129	195
Fair value, continuous unrealized loss position for less than 12 months	$ 544	$ 654
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (30)	$ (56)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	17	46
Fair value, continuous unrealized loss position for 12 months or more	$ 48	$ 165
Unrealized losses, continuous unrealized loss position for 12 months or more	(10)	(29)
Total unrealized losses
Total unrealized losses	$ (40)	$ (85)
Investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	1,689	2,202
Fair value, continuous unrealized loss position for less than 12 months	$ 12,345	$ 15,678
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (96)	$ (293)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	323	201
Fair value, continuous unrealized loss position for 12 months or more	$ 1,530	$ 493
Unrealized losses, continuous unrealized loss position for 12 months or more	(53)	(51)
Total unrealized losses
Total unrealized losses	$ (149)	$ (344)
Below investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | contract	74	193
Fair value, continuous unrealized loss position for less than 12 months	$ 622	$ 2,240
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (16)	$ (58)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more | contract	51	87
Fair value, continuous unrealized loss position for 12 months or more	$ 211	$ 429
Unrealized losses, continuous unrealized loss position for 12 months or more	(26)	(45)
Total unrealized losses
Total unrealized losses	$ (42)	$ (103)