Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Mar. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Less than 12 months
Fair value	$ 530	$ 1,643
Unrealized loss	(6)	(34)
12 months or more
Fair Value	1,118	1,952
Unrealized loss	(59)	(105)
Total
Fair value	1,648	3,595
Unrealized loss	$ (65)	$ (139)
Number of securities
Less than 12 months (in securities) | Security	128	417
12 months or more (in securities) | Security	333	608
Total (in securities) | Security	456	997
Number of securities with OTTI
Less than 12 months (in securities) | Security	5	22
12 months or more (in securities) | Security	24	22
Total (in securities) | Security	28	42
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 17	$ 195
Unrealized loss	0	(4)
12 months or more
Fair Value	152	658
Unrealized loss	(3)	(14)
Total
Fair value	169	853
Unrealized loss	(3)	(18)
US government and agencies [Member]
Less than 12 months
Fair value	20	11
Unrealized loss	0	0
12 months or more
Fair Value	24	24
Unrealized loss	0	(1)
Total
Fair value	44	35
Unrealized loss	0	(1)
Corporate securities [Member]
Less than 12 months
Fair value	101	836
Unrealized loss	(1)	(19)
12 months or more
Fair Value	381	522
Unrealized loss	(16)	(33)
Total
Fair value	482	1,358
Unrealized loss	(17)	(52)
RMBS [Member]
Less than 12 months
Fair value	37	85
Unrealized loss	(1)	(2)
12 months or more
Fair Value	371	447
Unrealized loss	(23)	(32)
Total
Fair value	408	532
Unrealized loss	(24)	(34)
CMBS [Member]
Less than 12 months
Fair value	1	111
Unrealized loss	0	(1)
12 months or more
Fair Value	86	164
Unrealized loss	(3)	(6)
Total
Fair value	87	275
Unrealized loss	(3)	(7)
Asset-backed Securities [Member]
Less than 12 months
Fair value	308	322
Unrealized loss	(3)	(4)
12 months or more
Fair Value	18	38
Unrealized loss	0	(1)
Total
Fair value	326	360
Unrealized loss	(3)	(5)
Foreign government securities [Member]
Less than 12 months
Fair value	46	83
Unrealized loss	(1)	(4)
12 months or more
Fair Value	86	99
Unrealized loss	(14)	(18)
Total
Fair value	132	182
Unrealized loss	$ (15)	$ (22)