Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Jun. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Less than 12 months
Fair value	$ 524	$ 1,643
Unrealized loss	(5)	(34)
12 months or more
Fair Value	765	1,952
Unrealized loss	(46)	(105)
Total
Fair value	1,289	3,595
Unrealized loss	$ (51)	$ (139)
Number of securities
Less than 12 months (in securities) | Security	112	417
12 months or more (in securities) | Security	186	608
Total (in securities) | Security	291	997
Number of securities with OTTI
Less than 12 months (in securities) | Security	6	22
12 months or more (in securities) | Security	11	22
Total (in securities) | Security	17	42
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 7	$ 195
Unrealized loss	0	(4)
12 months or more
Fair Value	17	658
Unrealized loss	0	(14)
Total
Fair value	24	853
Unrealized loss	0	(18)
US government and agencies [Member]
Less than 12 months
Fair value	10	11
Unrealized loss	0	0
12 months or more
Fair Value	21	24
Unrealized loss	0	(1)
Total
Fair value	31	35
Unrealized loss	0	(1)
Corporate securities [Member]
Less than 12 months
Fair value	229	836
Unrealized loss	(2)	(19)
12 months or more
Fair Value	204	522
Unrealized loss	(14)	(33)
Total
Fair value	433	1,358
Unrealized loss	(16)	(52)
RMBS [Member]
Less than 12 months
Fair value	7	85
Unrealized loss	0	(2)
12 months or more
Fair Value	283	447
Unrealized loss	(13)	(32)
Total
Fair value	290	532
Unrealized loss	(13)	(34)
CMBS [Member]
Less than 12 months
Fair value	0	111
Unrealized loss	0	(1)
12 months or more
Fair Value	51	164
Unrealized loss	(1)	(6)
Total
Fair value	51	275
Unrealized loss	(1)	(7)
Asset-backed Securities [Member]
Less than 12 months
Fair value	226	322
Unrealized loss	(2)	(4)
12 months or more
Fair Value	76	38
Unrealized loss	(1)	(1)
Total
Fair value	302	360
Unrealized loss	(3)	(5)
Foreign government securities [Member]
Less than 12 months
Fair value	45	83
Unrealized loss	(1)	(4)
12 months or more
Fair Value	113	99
Unrealized loss	(17)	(18)
Total
Fair value	158	182
Unrealized loss	$ (18)	$ (22)