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, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract
U.S. government and agencies
Less than 12 months
Number of issues | contract	9	11
Fair value	$ 112	$ 55
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | contract	39	38
Fair value	$ 259	$ 364
Unrealized losses	(2)	(6)
Total unrealized losses
Total unrealized losses	$ (2)	$ (6)
Municipal
Less than 12 months
Number of issues | contract	102	943
Fair value	$ 208	$ 1,633
Unrealized losses	$ 0	$ (10)
12 months or more
Number of issues | contract	600	1,147
Fair value	$ 742	$ 1,554
Unrealized losses	(8)	(33)
Total unrealized losses
Total unrealized losses	$ (8)	$ (43)
Corporate
Less than 12 months
Number of issues | contract	304	1,735
Fair value	$ 3,249	$ 19,243
Unrealized losses	$ (42)	$ (543)
12 months or more
Number of issues | contract	807	645
Fair value	$ 10,382	$ 8,374
Unrealized losses	(200)	(347)
Total unrealized losses
Total unrealized losses	$ (242)	$ (890)
Foreign government
Less than 12 months
Number of issues | contract	0	7
Fair value	$ 0	$ 20
Unrealized losses	$ 0	$ (1)
12 months or more
Number of issues | contract	11	27
Fair value	$ 218	$ 412
Unrealized losses	(1)	(4)
Total unrealized losses
Total unrealized losses	$ (1)	$ (5)
Asset-backed securities (“ABS”)
Less than 12 months
Number of issues | contract	36	64
Fair value	$ 283	$ 454
Unrealized losses	$ (4)	$ (5)
12 months or more
Number of issues | contract	34	28
Fair value	$ 146	$ 161
Unrealized losses	(5)	(5)
Total unrealized losses
Total unrealized losses	$ (9)	$ (10)
Residential mortgage-backed securities (“RMBS”)
Less than 12 months
Number of issues | contract	81	166
Fair value	$ 12	$ 30
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | contract	194	195
Fair value	$ 49	$ 52
Unrealized losses	(1)	(2)
Total unrealized losses
Total unrealized losses	$ (1)	$ (2)
Commercial mortgage-backed securities (“CMBS”)
Less than 12 months
Number of issues | contract	4	3
Fair value	$ 11	$ 7
Unrealized losses	$ (1)	$ 0
12 months or more
Number of issues | contract	1	2
Fair value	$ 0	$ 0
Unrealized losses	0	(1)
Total unrealized losses
Total unrealized losses	$ (1)	$ (1)
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	536	2,930
Fair value	$ 3,875	$ 21,442
Unrealized losses	$ (47)	$ (559)
12 months or more
Number of issues | contract	1,686	2,082
Fair value	$ 11,796	$ 10,917
Unrealized losses	(217)	(398)
Total unrealized losses
Total unrealized losses	$ (264)	$ (957)
Investment grade fixed income securities
Less than 12 months
Number of issues | contract	404	2,348
Fair value	$ 3,077	$ 17,485
Unrealized losses	$ (22)	$ (331)
12 months or more
Number of issues | contract	1,550	2,021
Fair value	$ 10,853	$ 10,626
Unrealized losses	(170)	(360)
Total unrealized losses
Total unrealized losses	$ (192)	$ (691)
Below investment grade fixed income securities
Less than 12 months
Number of issues | contract	132	582
Fair value	$ 798	$ 3,957
Unrealized losses	$ (25)	$ (228)
12 months or more
Number of issues | contract	136	61
Fair value	$ 943	$ 291
Unrealized losses	(47)	(38)
Total unrealized losses
Total unrealized losses	$ (72)	$ (266)