Investments - Continuous unrealized loss positions (Details) $ in Millions	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
U.S. government and agencies
Less than 12 months
Number of issues | security	66	46
Fair value	$ 2,829	$ 943
Unrealized losses	$ (18)	$ (9)
12 months or more
Number of issues | security	18	0
Fair value	$ 182	$ 0
Unrealized losses	(2)	0
Total unrealized losses	$ (20)	$ (9)
Municipal
Less than 12 months
Number of issues | security	1,756	1,310
Fair value	$ 3,143	$ 3,073
Unrealized losses	$ (24)	$ (76)
12 months or more
Number of issues | security	165	8
Fair value	$ 349	$ 29
Unrealized losses	(12)	(11)
Total unrealized losses	$ (36)	$ (87)
Corporate
Less than 12 months
Number of issues | security	781	862
Fair value	$ 11,616	$ 13,343
Unrealized losses	$ (102)	$ (256)
12 months or more
Number of issues | security	208	83
Fair value	$ 3,289	$ 678
Unrealized losses	(102)	(63)
Total unrealized losses	$ (204)	$ (319)
Foreign government
Less than 12 months
Number of issues | security	45	41
Fair value	$ 580	$ 225
Unrealized losses	$ (10)	$ (4)
12 months or more
Number of issues | security	5	0
Fair value	$ 44	$ 0
Unrealized losses	(1)	0
Total unrealized losses	$ (11)	$ (4)
ABS
Less than 12 months
Number of issues | security	57	31
Fair value	$ 476	$ 222
Unrealized losses	$ (3)	$ (1)
12 months or more
Number of issues | security	9	14
Fair value	$ 34	$ 109
Unrealized losses	(4)	(10)
Total unrealized losses	$ (7)	$ (11)
RMBS
Less than 12 months
Number of issues | security	118	89
Fair value	$ 35	$ 53
Unrealized losses	$ (1)	$ (1)
12 months or more
Number of issues | security	181	179
Fair value	$ 50	$ 91
Unrealized losses	(2)	(7)
Total unrealized losses	$ (3)	$ (8)
CMBS
Less than 12 months
Number of issues | security	2	15
Fair value	$ 1	$ 59
Unrealized losses	$ 0	$ (4)
12 months or more
Number of issues | security	6	4
Fair value	$ 23	$ 15
Unrealized losses	(2)	(5)
Total unrealized losses	$ (2)	$ (9)
Redeemable preferred stock
Less than 12 months
Number of issues | security	1	1
Fair value	$ 0	$ 0
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | security	0	0
Fair value	$ 0	$ 0
Unrealized losses	0	0
Total unrealized losses	$ 0	$ 0
Fixed income securities
Less than 12 months
Number of issues | security	2,826	2,395
Fair value	$ 18,680	$ 17,918
Unrealized losses	$ (158)	$ (351)
12 months or more
Number of issues | security	592	288
Fair value	$ 3,971	$ 922
Unrealized losses	(125)	(96)
Total unrealized losses	$ (283)	$ (447)
Equity securities
Less than 12 months
Number of issues | security	127	195
Fair value	$ 369	$ 654
Unrealized losses	$ (12)	$ (56)
12 months or more
Number of issues | security	2	46
Fair value	$ 0	$ 165
Unrealized losses	0	(29)
Total unrealized losses	$ (12)	$ (85)
Fixed income and equity securities
Less than 12 months
Number of issues | security	2,953	2,590
Fair value	$ 19,049	$ 18,572
Unrealized losses	$ (170)	$ (407)
12 months or more
Number of issues | security	594	334
Fair value	$ 3,971	$ 1,087
Unrealized losses	(125)	(125)
Total unrealized losses	$ (295)	$ (532)
Investment grade fixed income securities
Less than 12 months
Number of issues | security	2,706	2,202
Fair value	$ 17,668	$ 15,678
Unrealized losses	$ (134)	$ (293)
12 months or more
Number of issues | security	535	201
Fair value	$ 3,751	$ 493
Unrealized losses	(98)	(51)
Total unrealized losses	$ (232)	$ (344)
Below investment grade fixed income securities
Less than 12 months
Number of issues | security	120	193
Fair value	$ 1,012	$ 2,240
Unrealized losses	$ (24)	$ (58)
12 months or more
Number of issues | security	57	87
Fair value	$ 220	$ 429
Unrealized losses	(27)	(45)
Total unrealized losses	$ (51)	$ (103)