Investments - Portfolio Monitoring (Details) $ in Millions	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fixed income and equity securities
Total unrealized losses
Total unrealized losses	$ (129)
Fixed income securities
Less than 12 months
Number of issues | security	290	1,458
Fair value	$ 1,033	$ 7,088
Unrealized losses	$ (19)	$ (220)
12 months or more
Number of issues | security	589	436
Fair value	$ 3,597	$ 2,701
Unrealized losses	(110)	(169)
Total unrealized losses
Total unrealized losses	$ (129)	$ (389)
U.S. government and agencies
Less than 12 months
Number of issues | security	1	2
Fair value	$ 3	$ 6
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | security	0	1
Fair value	$ 0	$ 1
Unrealized losses	0	0
Total unrealized losses
Total unrealized losses	$ 0	$ 0
Municipal
Less than 12 months
Number of issues | security	3	38
Fair value	$ 25	$ 98
Unrealized losses	$ 0	$ (1)
12 months or more
Number of issues | security	2	5
Fair value	$ 17	$ 26
Unrealized losses	(3)	(3)
Total unrealized losses
Total unrealized losses	$ (3)	$ (4)
Corporate
Less than 12 months
Number of issues | security	219	1,260
Fair value	$ 932	$ 6,799
Unrealized losses	$ (17)	$ (218)
12 months or more
Number of issues | security	521	370
Fair value	$ 3,543	$ 2,633
Unrealized losses	(105)	(163)
Total unrealized losses
Total unrealized losses	$ (122)	$ (381)
ABS
Less than 12 months
Number of issues | security	12	30
Fair value	$ 58	$ 167
Unrealized losses	$ (1)	$ (1)
12 months or more
Number of issues | security	12	11
Fair value	$ 27	$ 31
Unrealized losses	(2)	(2)
Total unrealized losses
Total unrealized losses	$ (3)	$ (3)
RMBS
Less than 12 months
Number of issues | security	51	124
Fair value	$ 5	$ 11
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | security	53	47
Fair value	$ 10	$ 10
Unrealized losses	0	(1)
Total unrealized losses
Total unrealized losses	$ 0	$ (1)
CMBS
Less than 12 months
Number of issues | security	4	3
Fair value	$ 10	$ 7
Unrealized losses	$ (1)	$ 0
12 months or more
Number of issues | security	1	2
Fair value	$ 0	$ 0
Unrealized losses	0	0
Total unrealized losses
Total unrealized losses	$ (1)	$ 0
Redeemable preferred stock
Less than 12 months
Number of issues | security	0	1
Fair value	$ 0	$ 0
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | security	0	0
Fair value	$ 0	$ 0
Unrealized losses	0	0
Total unrealized losses
Total unrealized losses	$ 0	$ 0
Investment grade fixed income securities
Less than 12 months
Number of issues | security	175	948
Fair value	$ 639	$ 5,255
Unrealized losses	$ (8)	$ (121)
12 months or more
Number of issues | security	479	388
Fair value	$ 3,162	$ 2,551
Unrealized losses	(85)	(147)
Total unrealized losses
Total unrealized losses	$ (93)	$ (268)
Below investment grade fixed income securities
Less than 12 months
Number of issues | security	115	510
Fair value	$ 394	$ 1,833
Unrealized losses	$ (11)	$ (99)
12 months or more
Number of issues | security	110	48
Fair value	$ 435	$ 150
Unrealized losses	(25)	(22)
Total unrealized losses
Total unrealized losses	$ (36)	$ (121)