Investments - Gross unrealized losses and fair value by the type and length of time held in continuous unrealized loss position (Details) $ in Millions	Jun. 30, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract
U.S. government and agencies
Less than 12 months
Number of issues | contract	67	66
Fair value	$ 2,022	$ 2,829
Unrealized losses	$ (15)	$ (18)
12 months or more
Number of issues | contract	11	18
Fair value	$ 55	$ 182
Unrealized losses	(1)	(2)
Total unrealized losses
Total unrealized losses	$ (16)	$ (20)
Municipal
Less than 12 months
Number of issues | contract	2,456	1,756
Fair value	$ 4,039	$ 3,143
Unrealized losses	$ (57)	$ (24)
12 months or more
Number of issues | contract	151	165
Fair value	$ 268	$ 349
Unrealized losses	(14)	(12)
Total unrealized losses
Total unrealized losses	$ (71)	$ (36)
Corporate
Less than 12 months
Number of issues | contract	1,888	781
Fair value	$ 24,450	$ 11,616
Unrealized losses	$ (603)	$ (102)
12 months or more
Number of issues | contract	240	208
Fair value	$ 2,878	$ 3,289
Unrealized losses	(156)	(102)
Total unrealized losses
Total unrealized losses	$ (759)	$ (204)
Foreign government
Less than 12 months
Number of issues | contract	15	45
Fair value	$ 84	$ 580
Unrealized losses	$ 0	$ (10)
12 months or more
Number of issues | contract	27	5
Fair value	$ 448	$ 44
Unrealized losses	(9)	(1)
Total unrealized losses
Total unrealized losses	$ (9)	$ (11)
Asset-backed securities (“ABS”)
Less than 12 months
Number of issues | contract	76	57
Fair value	$ 505	$ 476
Unrealized losses	$ (5)	$ (3)
12 months or more
Number of issues | contract	8	9
Fair value	$ 13	$ 34
Unrealized losses	(3)	(4)
Total unrealized losses
Total unrealized losses	$ (8)	$ (7)
Residential mortgage-backed securities (“RMBS”)
Less than 12 months
Number of issues | contract	143	118
Fair value	$ 35	$ 35
Unrealized losses	$ (1)	$ (1)
12 months or more
Number of issues | contract	181	181
Fair value	$ 40	$ 50
Unrealized losses	(1)	(2)
Total unrealized losses
Total unrealized losses	$ (2)	$ (3)
Commercial mortgage-backed securities (“CMBS”)
Less than 12 months
Number of issues | contract	2	2
Fair value	$ 11	$ 1
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | contract	5	6
Fair value	$ 10	$ 23
Unrealized losses	(2)	(2)
Total unrealized losses
Total unrealized losses	$ (2)	$ (2)
Redeemable preferred stock
Less than 12 months
Number of issues | contract	1	1
Fair value	$ 0	$ 0
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | contract	0	0
Fair value	$ 0	$ 0
Unrealized losses	0	0
Total unrealized losses
Total unrealized losses	$ 0	$ 0
Fixed income securities
Less than 12 months
Number of issues | contract	4,648	2,826
Fair value	$ 31,146	$ 18,680
Unrealized losses	$ (681)	$ (158)
12 months or more
Number of issues | contract	623	592
Fair value	$ 3,712	$ 3,971
Unrealized losses	(186)	(125)
Total unrealized losses
Total unrealized losses	(867)	$ (283)
Equity securities
Less than 12 months
Number of issues | contract		127
Fair value		$ 369
Unrealized losses		$ (12)
12 months or more
Number of issues | contract		2
Fair value		$ 0
Unrealized losses		0
Total unrealized losses
Total unrealized losses		$ (12)
Fixed income and equity securities
Less than 12 months
Number of issues | contract		2,953
Fair value		$ 19,049
Unrealized losses		$ (170)
12 months or more
Number of issues | contract		594
Fair value		$ 3,971
Unrealized losses		(125)
Total unrealized losses
Total unrealized losses	$ (867)	$ (295)
Investment grade fixed income securities
Less than 12 months
Number of issues | contract	4,218	2,706
Fair value	$ 27,970	$ 17,668
Unrealized losses	$ (573)	$ (134)
12 months or more
Number of issues | contract	575	535
Fair value	$ 3,537	$ 3,751
Unrealized losses	(168)	(98)
Total unrealized losses
Total unrealized losses	$ (741)	$ (232)
Below investment grade fixed income securities
Less than 12 months
Number of issues | contract	430	120
Fair value	$ 3,176	$ 1,012
Unrealized losses	$ (108)	$ (24)
12 months or more
Number of issues | contract	48	57
Fair value	$ 175	$ 220
Unrealized losses	(18)	(27)
Total unrealized losses
Total unrealized losses	$ (126)	$ (51)