Investments - Portfolio Monitoring (Details) $ in Millions	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Fixed income and equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security		670
Fair value, continuous unrealized loss position for less than 12 months		$ 3,596
Unrealized losses, continuous unrealized loss position for less than 12 months		$ (34)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security		218
Fair value, continuous unrealized loss position for 12 months or more		$ 1,425
Unrealized losses, continuous unrealized loss position for 12 months or more		(67)
Total unrealized losses
Total unrealized losses	$ (252)	$ (101)
Fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	1,282	583
Fair value, continuous unrealized loss position for less than 12 months	$ 7,107	$ 3,530
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (158)	$ (31)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	209	217
Fair value, continuous unrealized loss position for 12 months or more	$ 1,316	$ 1,425
Unrealized losses, continuous unrealized loss position for 12 months or more	(94)	(67)
Total unrealized losses
Gross unrealized losses	$ (252)	$ (98)
U.S. government and agencies
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	9	17
Fair value, continuous unrealized loss position for less than 12 months	$ 104	$ 443
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (1)	$ (2)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	1	2
Fair value, continuous unrealized loss position for 12 months or more	$ 5	$ 25
Unrealized losses, continuous unrealized loss position for 12 months or more	0	0
Total unrealized losses
Gross unrealized losses	$ (1)	$ (2)
Municipal
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	55	4
Fair value, continuous unrealized loss position for less than 12 months	$ 119	$ 14
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (2)	$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	1	1
Fair value, continuous unrealized loss position for 12 months or more	$ 11	$ 11
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(3)
Total unrealized losses
Gross unrealized losses	$ (4)	$ (3)
Corporate
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	1,082	456
Fair value, continuous unrealized loss position for less than 12 months	$ 6,649	$ 2,899
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (153)	$ (28)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	139	144
Fair value, continuous unrealized loss position for 12 months or more	$ 1,250	$ 1,324
Unrealized losses, continuous unrealized loss position for 12 months or more	(86)	(58)
Total unrealized losses
Gross unrealized losses	$ (239)	$ (86)
Foreign government
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	2
Fair value, continuous unrealized loss position for less than 12 months	$ 16
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	0
Fair value, continuous unrealized loss position for 12 months or more	$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more	0
Total unrealized losses
Gross unrealized losses	$ 0
ABS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	48	33
Fair value, continuous unrealized loss position for less than 12 months	$ 215	$ 170
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (2)	$ (1)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	6	8
Fair value, continuous unrealized loss position for 12 months or more	$ 10	$ 24
Unrealized losses, continuous unrealized loss position for 12 months or more	(3)	(3)
Total unrealized losses
Gross unrealized losses	$ (5)	$ (4)
RMBS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	86	70
Fair value, continuous unrealized loss position for less than 12 months	$ 4	$ 3
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0	$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	56	56
Fair value, continuous unrealized loss position for 12 months or more	$ 16	$ 18
Unrealized losses, continuous unrealized loss position for 12 months or more	(1)	(1)
Total unrealized losses
Gross unrealized losses	$ (1)	$ (1)
CMBS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	0	2
Fair value, continuous unrealized loss position for less than 12 months	$ 0	$ 1
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0	$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	6	6
Fair value, continuous unrealized loss position for 12 months or more	$ 24	$ 23
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(2)
Total unrealized losses
Gross unrealized losses	$ (2)	$ (2)
Redeemable preferred stock
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security		1
Fair value, continuous unrealized loss position for less than 12 months		$ 0
Unrealized losses, continuous unrealized loss position for less than 12 months		$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security		0
Fair value, continuous unrealized loss position for 12 months or more		$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more		0
Total unrealized losses
Gross unrealized losses		$ 0
Equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security		87
Fair value, continuous unrealized loss position for less than 12 months		$ 66
Unrealized losses, continuous unrealized loss position for less than 12 months		$ (3)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security		1
Fair value, continuous unrealized loss position for 12 months or more		$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more		0
Total unrealized losses
Gross unrealized losses on equity securities		$ (3)
Investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	960	472
Fair value, continuous unrealized loss position for less than 12 months	$ 6,028	$ 3,192
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (123)	$ (22)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	177	181
Fair value, continuous unrealized loss position for 12 months or more	$ 1,211	$ 1,320
Unrealized losses, continuous unrealized loss position for 12 months or more	(79)	(52)
Total unrealized losses
Total unrealized losses	$ (202)	$ (74)
Below investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	322	111
Fair value, continuous unrealized loss position for less than 12 months	$ 1,079	$ 338
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (35)	$ (9)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	32	36
Fair value, continuous unrealized loss position for 12 months or more	$ 105	$ 105
Unrealized losses, continuous unrealized loss position for 12 months or more	(15)	(15)
Total unrealized losses
Total unrealized losses	$ (50)	$ (24)