Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Mar. 31, 2019 USD ($) securities	Dec. 31, 2018 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 17	$ 95
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	23	264
More than twelve months below amortized cost, Unrealized Capital Loss	157	201
Total Unrealized Capital Losses	197	560
Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Amortized Cost	1,405	4,531
Six months or less below amortized cost, Unrealized Capital Loss	$ 34	$ 106
Six months or less below amortized cost, Number of Securities | securities	277	826
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 1,108	$ 5,535
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 24	$ 235
More than six months and twelve months or less below amortized cost, Number of Securities | securities	218	1,063
More than twelve months below amortized cost, Amortized Cost	$ 3,509	$ 2,378
More than twelve months below amortized cost, Unrealized Capital Loss	$ 106	$ 144
More than twelve months below amortized cost, Number of Securities | securities	743	519
Total Amortized Cost	$ 6,022	$ 12,444
Total Unrealized Capital Losses	$ 164	$ 485
Total Number of Securities | securities	1,238	2,408
Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Amortized Cost	$ 22	$ 88
Six months or less below amortized cost, Unrealized Capital Loss	$ 5	$ 21
Six months or less below amortized cost, Number of Securities | securities	10	25
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 0	$ 73
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 0	$ 27
More than six months and twelve months or less below amortized cost, Number of Securities | securities	6	6
More than twelve months below amortized cost, Amortized Cost	$ 94	$ 80
More than twelve months below amortized cost, Unrealized Capital Loss	$ 28	$ 27
More than twelve months below amortized cost, Number of Securities | securities	5	5
Total Amortized Cost	$ 116	$ 241
Total Unrealized Capital Losses	$ 33	$ 75
Total Number of Securities | securities	21	36
U.S. Treasuries
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 0	$ 0
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	0	0
More than twelve months below amortized cost, Unrealized Capital Loss	0	0
Total Unrealized Capital Losses	0	0
U.S. Treasuries | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	15	15
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	5	5
U.S. Treasuries | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 0
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	0	0
State, municipalities and political subdivisions
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 0	$ 0
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	0	3
More than twelve months below amortized cost, Unrealized Capital Loss	3	5
Total Unrealized Capital Losses	3	8
State, municipalities and political subdivisions | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	99	287
Total Unrealized Capital Losses	$ 3	$ 8
Total Number of Securities | securities	51	132
State, municipalities and political subdivisions | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 0
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	0	0
U.S. corporate public securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 3	$ 37
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	6	94
More than twelve months below amortized cost, Unrealized Capital Loss	46	50
Total Unrealized Capital Losses	55	181
U.S. corporate public securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,328	3,721
Total Unrealized Capital Losses	$ 46	$ 164
Total Number of Securities | securities	286	796
U.S. corporate public securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 25	$ 55
Total Unrealized Capital Losses	$ 9	$ 17
Total Number of Securities | securities	3	8
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 3	$ 13
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	0	27
More than twelve months below amortized cost, Unrealized Capital Loss	39	66
Total Unrealized Capital Losses	42	106
U.S. corporate private securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	894	2,120
Total Unrealized Capital Losses	$ 24	$ 84
Total Number of Securities | securities	105	245
U.S. corporate private securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 66	$ 67
Total Unrealized Capital Losses	$ 18	$ 22
Total Number of Securities | securities	2	2
Foreign corporate public securities and foreign governments
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 1	$ 12
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	3	42
More than twelve months below amortized cost, Unrealized Capital Loss	28	26
Total Unrealized Capital Losses	32	80
Foreign corporate public securities and foreign governments | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	656	1,348
Total Unrealized Capital Losses	$ 26	$ 65
Total Number of Securities | securities	132	307
Foreign corporate public securities and foreign governments | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 23	$ 60
Total Unrealized Capital Losses	$ 6	$ 15
Total Number of Securities | securities	6	9
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 0	$ 10
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	4	67
More than twelve months below amortized cost, Unrealized Capital Loss	13	20
Total Unrealized Capital Losses	17	97
Foreign corporate private securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	690	1,765
Total Unrealized Capital Losses	$ 17	$ 76
Total Number of Securities | securities	64	157
Foreign corporate private securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 57
Total Unrealized Capital Losses	$ 0	$ 21
Total Number of Securities | securities	0	6
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 4	$ 6
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	0	5
More than twelve months below amortized cost, Unrealized Capital Loss	14	21
Total Unrealized Capital Losses	18	32
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	837	1,004
Total Unrealized Capital Losses	$ 18	$ 32
Total Number of Securities | securities	250	301
Residential mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 2	$ 0
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	10	8
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 2	$ 6
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	1	10
More than twelve months below amortized cost, Unrealized Capital Loss	11	12
Total Unrealized Capital Losses	14	28
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	685	1,205
Total Unrealized Capital Losses	$ 14	$ 28
Total Number of Securities | securities	129	228
Commercial mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 0
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	0	0
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 4	$ 11
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	9	16
More than twelve months below amortized cost, Unrealized Capital Loss	3	1
Total Unrealized Capital Losses	16	28
Other asset-backed securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	818	979
Total Unrealized Capital Losses	$ 16	$ 28
Total Number of Securities | securities	216	237
Other asset-backed securities | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 2
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	0	3