Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Sep. 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	$ 87	$ 63
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	179	3
More than twelve months below amortized cost, Unrealized Capital Loss	180	82
Total Unrealized Capital Losses	446	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	4,762	1,883
Six months or less below amortized cost, Unrealized Capital Loss	$ 71	$ 30
Six months or less below amortized cost, Number of Securities | securities	918	433
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 4,548	$ 128
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 178	$ 4
More than six months and twelve months or less below amortized cost, Number of Securities | securities	921	37
More than twelve months below amortized cost, Amortized Cost	$ 1,999	$ 1,661
More than twelve months below amortized cost, Unrealized Capital Loss	$ 148	$ 53
More than twelve months below amortized cost, Number of Securities | securities	402	335
Total Amortized Cost	$ 11,309	$ 3,672
Total Unrealized Capital Losses	$ 397	$ 87
Total Number of Securities | securities	2,241	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	$ 75	$ 67
Six months or less below amortized cost, Unrealized Capital Loss	$ 24	$ 38
Six months or less below amortized cost, Number of Securities | securities	15	7
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 15	$ 7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 4	$ 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	$ 80	$ 72
More than twelve months below amortized cost, Unrealized Capital Loss	$ 21	$ 21
More than twelve months below amortized cost, Number of Securities | securities	5	7
Total Amortized Cost	$ 170	$ 146
Total Unrealized Capital Losses	$ 49	$ 61
Total Number of Securities | securities	21	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	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	30
Total Unrealized Capital Losses	$ 0	$ 0
Total Number of Securities | securities	5	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	$ 4	$ 0
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	4	0
More than twelve months below amortized cost, Unrealized Capital Loss	6	4
Total Unrealized Capital Losses	14	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	422	130
Total Unrealized Capital Losses	$ 14	$ 4
Total Number of Securities | securities	184	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	$ 20	$ 11
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	59	0
More than twelve months below amortized cost, Unrealized Capital Loss	37	15
Total Unrealized Capital Losses	116	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	3,197	828
Total Unrealized Capital Losses	$ 111	$ 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	$ 21	$ 6
Total Unrealized Capital Losses	$ 5	$ 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	$ 6	$ 2
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	27	2
More than twelve months below amortized cost, Unrealized Capital Loss	63	34
Total Unrealized Capital Losses	96	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	2,017	743
Total Unrealized Capital Losses	$ 78	$ 18
Total Number of Securities | securities	220	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	$ 66	$ 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	$ 8	$ 1
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	30	0
More than twelve months below amortized cost, Unrealized Capital Loss	13	8
Total Unrealized Capital Losses	51	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	1,238	254
Total Unrealized Capital Losses	$ 48	$ 7
Total Number of Securities | securities	267	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	$ 13	$ 7
Total Unrealized Capital Losses	$ 3	$ 2
Total Number of Securities | securities	3	1
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 33	$ 38
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	24	0
More than twelve months below amortized cost, Unrealized Capital Loss	17	7
Total Unrealized Capital Losses	74	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	1,524	288
Total Unrealized Capital Losses	$ 54	$ 8
Total Number of Securities | securities	132	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	$ 20	$ 37
Total Number of Securities | securities	3	6
Residential mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 4	$ 5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	12	1
More than twelve months below amortized cost, Unrealized Capital Loss	30	13
Total Unrealized Capital Losses	46	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	924	746
Total Unrealized Capital Losses	$ 43	$ 19
Total Number of Securities | securities	368	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	$ 13	$ 0
Total Unrealized Capital Losses	$ 3	$ 0
Total Number of Securities | securities	10	3
Commercial mortgage-backed
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 8	$ 5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	21	0
More than twelve months below amortized cost, Unrealized Capital Loss	13	1
Total Unrealized Capital Losses	42	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	1,251	511
Total Unrealized Capital Losses	$ 42	$ 6
Total Number of Securities | securities	238	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	$ 4	$ 1
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	2	0
More than twelve months below amortized cost, Unrealized Capital Loss	1	0
Total Unrealized Capital Losses	7	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	721	142
Total Unrealized Capital Losses	$ 7	$ 1
Total Number of Securities | securities	172	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