Investments - Gross unrealized losses and fair value by the type and length of time held in continuous unrealized loss position (Details) $ in Millions	Sep. 30, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract
U.S. government and agencies
Less than 12 months
Number of issues | contract	65	66
Fair value	$ 2,517	$ 2,829
Unrealized losses	$ (23)	$ (18)
12 months or more
Number of issues | contract	26	18
Fair value	$ 175	$ 182
Unrealized losses	(4)	(2)
Total unrealized losses
Total unrealized losses	$ (27)	$ (20)
Municipal
Less than 12 months
Number of issues | contract	3,192	1,756
Fair value	$ 5,600	$ 3,143
Unrealized losses	$ (75)	$ (24)
12 months or more
Number of issues | contract	480	165
Fair value	$ 667	$ 349
Unrealized losses	(30)	(12)
Total unrealized losses
Total unrealized losses	$ (105)	$ (36)
Corporate
Less than 12 months
Number of issues | contract	1,823	781
Fair value	$ 24,061	$ 11,616
Unrealized losses	$ (500)	$ (102)
12 months or more
Number of issues | contract	329	208
Fair value	$ 4,274	$ 3,289
Unrealized losses	(223)	(102)
Total unrealized losses
Total unrealized losses	$ (723)	$ (204)
Foreign government
Less than 12 months
Number of issues | contract	26	45
Fair value	$ 166	$ 580
Unrealized losses	$ (2)	$ (10)
12 months or more
Number of issues | contract	25	5
Fair value	$ 432	$ 44
Unrealized losses	(10)	(1)
Total unrealized losses
Total unrealized losses	$ (12)	$ (11)
Asset-backed securities (“ABS”)
Less than 12 months
Number of issues | contract	68	57
Fair value	$ 442	$ 476
Unrealized losses	$ (3)	$ (3)
12 months or more
Number of issues | contract	19	9
Fair value	$ 107	$ 34
Unrealized losses	(5)	(4)
Total unrealized losses
Total unrealized losses	$ (8)	$ (7)
Residential mortgage-backed securities (“RMBS”)
Less than 12 months
Number of issues | contract	97	118
Fair value	$ 21	$ 35
Unrealized losses	$ 0	$ (1)
12 months or more
Number of issues | contract	182	181
Fair value	$ 53	$ 50
Unrealized losses	(2)	(2)
Total unrealized losses
Total unrealized losses	$ (2)	$ (3)
Commercial mortgage-backed securities (“CMBS”)
Less than 12 months
Number of issues | contract	5	2
Fair value	$ 18	$ 1
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | contract	3	6
Fair value	$ 1	$ 23
Unrealized losses	(1)	(2)
Total unrealized losses
Total unrealized losses	$ (1)	$ (2)
Redeemable preferred stock
Less than 12 months
Number of issues | contract		1
Fair value		$ 0
Unrealized losses		$ 0
12 months or more
Number of issues | contract		0
Fair value		$ 0
Unrealized losses		0
Total unrealized losses
Total unrealized losses		$ 0
Fixed income securities
Less than 12 months
Number of issues | contract	5,276	2,826
Fair value	$ 32,825	$ 18,680
Unrealized losses	$ (603)	$ (158)
12 months or more
Number of issues | contract	1,064	592
Fair value	$ 5,709	$ 3,971
Unrealized losses	(275)	(125)
Total unrealized losses
Total unrealized losses	(878)	$ (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	$ (878)	$ (295)
Investment grade fixed income securities
Less than 12 months
Number of issues | contract	4,939	2,706
Fair value	$ 30,338	$ 17,668
Unrealized losses	$ (529)	$ (134)
12 months or more
Number of issues | contract	1,015	535
Fair value	$ 5,461	$ 3,751
Unrealized losses	(253)	(98)
Total unrealized losses
Total unrealized losses	$ (782)	$ (232)
Below investment grade fixed income securities
Less than 12 months
Number of issues | contract	337	120
Fair value	$ 2,487	$ 1,012
Unrealized losses	$ (74)	$ (24)
12 months or more
Number of issues | contract	49	57
Fair value	$ 248	$ 220
Unrealized losses	(22)	(27)
Total unrealized losses
Total unrealized losses	$ (96)	$ (51)