Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Jun. 30, 2018 USD ($) securities	Dec. 31, 2017 USD ($) securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 197	$ 63
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	81	3
More than twelve months below amortized cost, Unrealized Capital Loss	128	82
Total Unrealized Capital Losses	406	148
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	7,813	1,883
Six months or less below amortized cost, Unrealized Capital Loss	$ 183	$ 30
Six months or less below amortized cost, Number of Securities | securities	1,568	433
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 1,434	$ 128
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 82	$ 4
More than six months and twelve months or less below amortized cost, Number of Securities | securities	310	37
More than twelve months below amortized cost, Amortized Cost	$ 1,308	$ 1,661
More than twelve months below amortized cost, Unrealized Capital Loss	$ 96	$ 53
More than twelve months below amortized cost, Number of Securities | securities	278	335
Total Amortized Cost	$ 10,555	$ 3,672
Total Unrealized Capital Losses	$ 361	$ 87
Total Number of Securities | securities	2,156	805
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	$ 82	$ 67
Six months or less below amortized cost, Unrealized Capital Loss	$ 23	$ 38
Six months or less below amortized cost, Number of Securities | securities	11	7
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 1	$ 7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 0	$ 2
More than six months and twelve months or less below amortized cost, Number of Securities | securities	1	1
More than twelve months below amortized cost, Amortized Cost	$ 79	$ 72
More than twelve months below amortized cost, Unrealized Capital Loss	$ 22	$ 21
More than twelve months below amortized cost, Number of Securities | securities	5	7
Total Amortized Cost	$ 162	$ 146
Total Unrealized Capital Losses	$ 45	$ 61
Total Number of Securities | securities	17	15
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	1	0
More than twelve months below amortized cost, Unrealized Capital Loss	0	0
Total Unrealized Capital Losses	1	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	28	30
Total Unrealized Capital Losses	$ 1	$ 0
Total Number of Securities | securities	6	6
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	$ 3	$ 0
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	1	0
More than twelve months below amortized cost, Unrealized Capital Loss	4	4
Total Unrealized Capital Losses	8	4
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	296	130
Total Unrealized Capital Losses	$ 8	$ 4
Total Number of Securities | securities	145	96
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	$ 70	$ 11
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	26	0
More than twelve months below amortized cost, Unrealized Capital Loss	23	15
Total Unrealized Capital Losses	119	26
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	3,084	828
Total Unrealized Capital Losses	$ 113	$ 24
Total Number of Securities | securities	655	167
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	$ 23	$ 6
Total Unrealized Capital Losses	$ 6	$ 2
Total Number of Securities | securities	2	2
U.S. corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 26	$ 2
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	11	2
More than twelve months below amortized cost, Unrealized Capital Loss	53	34
Total Unrealized Capital Losses	90	38
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	1,904	743
Total Unrealized Capital Losses	$ 72	$ 18
Total Number of Securities | securities	215	71
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	$ 67	$ 66
Total Unrealized Capital Losses	$ 18	$ 20
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	$ 32	$ 1
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	9	0
More than twelve months below amortized cost, Unrealized Capital Loss	12	8
Total Unrealized Capital Losses	53	9
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	1,215	254
Total Unrealized Capital Losses	$ 50	$ 7
Total Number of Securities | securities	276	61
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	$ 14	$ 7
Total Unrealized Capital Losses	$ 3	$ 2
Total Number of Securities | securities	4	1
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 46	$ 38
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	3	0
More than twelve months below amortized cost, Unrealized Capital Loss	16	7
Total Unrealized Capital Losses	65	45
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	1,336	288
Total Unrealized Capital Losses	$ 47	$ 8
Total Number of Securities | securities	122	35
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	$ 56	$ 66
Total Unrealized Capital Losses	$ 18	$ 37
Total Number of Securities | securities	4	6
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 7	$ 5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	14	1
More than twelve months below amortized cost, Unrealized Capital Loss	17	13
Total Unrealized Capital Losses	38	19
Residential mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	964	746
Total Unrealized Capital Losses	$ 38	$ 19
Total Number of Securities | securities	382	194
Residential mortgage-backed | Fair value decline below amortized cost greater than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	$ 1	$ 0
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	4	3
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 10	$ 5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	15	0
More than twelve months below amortized cost, Unrealized Capital Loss	2	1
Total Unrealized Capital Losses	27	6
Commercial mortgage-backed | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,106	511
Total Unrealized Capital Losses	$ 27	$ 6
Total Number of Securities | securities	217	131
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	$ 3	$ 1
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	1	0
More than twelve months below amortized cost, Unrealized Capital Loss	1	0
Total Unrealized Capital Losses	5	1
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	622	142
Total Unrealized Capital Losses	$ 5	$ 1
Total Number of Securities | securities	138	44
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	$ 1	$ 1
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	1	1