Investments - Unrealized Capital Losses 1 (Details) $ in Millions	Dec. 31, 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	$ 95	$ 63
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	264	3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	201	82
Total Unrealized Capital Losses	560	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	4,531	1,883
Six months or less below amortized cost, Unrealized Capital Loss	$ 106	$ 30
Six months or less below amortized cost, Number of Securities | securities	826	433
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 5,535	$ 128
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 235	$ 4
More than six months and twelve months or less below amortized cost, Number of Securities | securities	1,063	37
More than twelve months below amortized cost, Amortized Cost	$ 2,378	$ 1,661
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 144	$ 53
More than twelve months below amortized cost, Number of Securities | securities	519	335
Total Amortized Cost	$ 12,444	$ 3,672
Total Unrealized Capital Losses	$ 485	$ 87
Number of Securities | securities	2,408	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	$ 88	$ 67
Six months or less below amortized cost, Unrealized Capital Loss	$ 21	$ 38
Six months or less below amortized cost, Number of Securities | securities	25	7
More than six months and twelve months or less below amortized cost, Amortized Cost	$ 73	$ 7
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	$ 27	$ 2
More than six months and twelve months or less below amortized cost, Number of Securities | securities	6	1
More than twelve months below amortized cost, Amortized Cost	$ 80	$ 72
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	$ 27	$ 21
More than twelve months below amortized cost, Number of Securities | securities	5	7
Total Amortized Cost	$ 241	$ 146
Total Unrealized Capital Losses	$ 75	$ 61
Number of Securities | securities	36	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
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
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	3	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	5	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	287	130
Total Unrealized Capital Losses	$ 8	$ 4
Number of Securities | securities	132	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
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	$ 37	$ 11
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	94	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	50	15
Total Unrealized Capital Losses	181	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	3,721	828
Total Unrealized Capital Losses	$ 164	$ 24
Number of Securities | securities	796	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	$ 55	$ 6
Total Unrealized Capital Losses	$ 17	$ 2
Number of Securities | securities	8	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	$ 13	$ 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	66	34
Total Unrealized Capital Losses	106	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	2,120	743
Total Unrealized Capital Losses	$ 84	$ 18
Number of Securities | securities	245	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	$ 22	$ 20
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	$ 12	$ 1
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	42	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	26	8
Total Unrealized Capital Losses	80	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	1,348	254
Total Unrealized Capital Losses	$ 65	$ 7
Number of Securities | securities	307	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	$ 60	$ 7
Total Unrealized Capital Losses	$ 15	$ 2
Number of Securities | securities	9	1
Foreign corporate private securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 10	$ 38
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	67	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	20	7
Total Unrealized Capital Losses	97	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	1,765	288
Total Unrealized Capital Losses	$ 76	$ 8
Number of Securities | securities	157	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	$ 57	$ 66
Total Unrealized Capital Losses	$ 21	$ 37
Number of Securities | securities	6	6
Residential mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 6	$ 5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	5	1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	21	13
Total Unrealized Capital Losses	32	19
Residential mortgage-backed securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,004	746
Total Unrealized Capital Losses	$ 32	$ 19
Number of Securities | securities	301	194
Residential mortgage-backed securities | 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
Number of Securities | securities	8	3
Commercial mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Six months or less below amortized cost, Unrealized Capital Loss	$ 6	$ 5
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	10	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	12	1
Total Unrealized Capital Losses	28	6
Commercial mortgage-backed securities | Fair value decline below amortized cost less than 20%
Available-for-sale Securities Including Securities Pledged [Line Items]
Total Amortized Cost	1,205	511
Total Unrealized Capital Losses	$ 28	$ 6
Number of Securities | securities	228	131
Commercial mortgage-backed securities | 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
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	$ 11	$ 1
More than six months and twelve months or less below amortized cost, Unrealized Capital Loss	16	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Loss	1	0
Total Unrealized Capital Losses	28	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	979	142
Total Unrealized Capital Losses	$ 28	$ 1
Number of Securities | securities	237	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	$ 2	$ 1
Total Unrealized Capital Losses	$ 0	$ 0
Number of Securities | securities	3	1