Investments - Portfolio Monitoring (Details) $ in Millions	Jun. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fixed income securities
Gross Unrealized Loss
Fixed income securities with unrealized loss position less than 20% of amortized cost	$ 39
Fixed income securities with unrealized loss position greater than or equal to 20% of amortized cost	$ 16
Less than 12 months
Number of issues | security	142	1,458
Fair value	$ 370	$ 7,088
Unrealized losses	$ (11)	$ (220)
12 months or more
Number of issues | security	212	436
Fair value	$ 1,303	$ 2,701
Unrealized losses	(44)	(169)
Total unrealized losses
Total unrealized losses	$ (55)	$ (389)
U.S. government and agencies
Less than 12 months
Number of issues | security	0	2
Fair value	$ 0	$ 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	0	38
Fair value	$ 0	$ 98
Unrealized losses	$ 0	$ (1)
12 months or more
Number of issues | security	1	5
Fair value	$ 13	$ 26
Unrealized losses	(3)	(3)
Total unrealized losses
Total unrealized losses	$ (3)	$ (4)
Corporate
Less than 12 months
Number of issues | security	101	1,260
Fair value	$ 329	$ 6,799
Unrealized losses	$ (7)	$ (218)
12 months or more
Number of issues | security	175	370
Fair value	$ 1,278	$ 2,633
Unrealized losses	(39)	(163)
Total unrealized losses
Total unrealized losses	$ (46)	$ (381)
ABS
Less than 12 months
Number of issues | security	7	30
Fair value	$ 28	$ 167
Unrealized losses	$ (1)	$ (1)
12 months or more
Number of issues | security	4	11
Fair value	$ 8	$ 31
Unrealized losses	(2)	(2)
Total unrealized losses
Total unrealized losses	$ (3)	$ (3)
RMBS
Less than 12 months
Number of issues | security	31	124
Fair value	$ 5	$ 11
Unrealized losses	$ 0	$ 0
12 months or more
Number of issues | security	31	47
Fair value	$ 4	$ 10
Unrealized losses	0	(1)
Total unrealized losses
Total unrealized losses	$ 0	$ (1)
CMBS
Less than 12 months
Number of issues | security	2	3
Fair value	$ 8	$ 7
Unrealized losses	$ (3)	$ 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	$ (3)	$ 0
Redeemable preferred stock
Less than 12 months
Number of issues | security	1	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
Gross Unrealized Loss
Fixed income securities with unrealized loss position less than 20% of amortized cost	20
Fixed income securities with unrealized loss position greater than or equal to 20% of amortized cost	$ 12
Less than 12 months
Number of issues | security	63	948
Fair value	$ 133	$ 5,255
Unrealized losses	$ (1)	$ (121)
12 months or more
Number of issues | security	162	388
Fair value	$ 1,108	$ 2,551
Unrealized losses	(31)	(147)
Total unrealized losses
Total unrealized losses	(32)	$ (268)
Below investment grade fixed income securities
Gross Unrealized Loss
Fixed income securities with unrealized loss position less than 20% of amortized cost	19
Fixed income securities with unrealized loss position greater than or equal to 20% of amortized cost	$ 4
Less than 12 months
Number of issues | security	79	510
Fair value	$ 237	$ 1,833
Unrealized losses	$ (10)	$ (99)
12 months or more
Number of issues | security	50	48
Fair value	$ 195	$ 150
Unrealized losses	(13)	(22)
Total unrealized losses
Total unrealized losses	$ (23)	$ (121)