Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Less than 12 months
Fair value	$ 2,390	$ 806
Unrealized loss	(44)	(10)
12 months or more
Fair Value	906	975
Unrealized loss	(65)	(57)
Total
Fair value	3,296	1,781
Unrealized loss	$ (109)	$ (67)
Number of securities
Less than 12 months (in securities) | Security	716	244
12 months or more (in securities) | Security	244	264
Total (in securities) | Security	944	499
Number of securities with OTTI
Less than 12 months (in securities) | Security	20	17
12 months or more (in securities) | Security	16	15
Total (in securities) | Security	35	31
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 988	$ 166
Unrealized loss	(15)	(4)
12 months or more
Fair Value	260	281
Unrealized loss	(12)	(7)
Total
Fair value	1,248	447
Unrealized loss	(27)	(11)
US government and agencies [Member]
Less than 12 months
Fair value	62	151
Unrealized loss	0	0
12 months or more
Fair Value	17	18
Unrealized loss	(1)	(1)
Total
Fair value	79	169
Unrealized loss	(1)	(1)
Corporate securities [Member]
Less than 12 months
Fair value	692	201
Unrealized loss	(14)	(1)
12 months or more
Fair Value	226	240
Unrealized loss	(18)	(17)
Total
Fair value	918	441
Unrealized loss	(32)	(18)
RMBS [Member]
Less than 12 months
Fair value	350	191
Unrealized loss	(11)	(5)
12 months or more
Fair Value	203	213
Unrealized loss	(17)	(12)
Total
Fair value	553	404
Unrealized loss	(28)	(17)
CMBS [Member]
Less than 12 months
Fair value	222	29
Unrealized loss	(3)	0
12 months or more
Fair Value	78	80
Unrealized loss	(4)	(3)
Total
Fair value	300	109
Unrealized loss	(7)	(3)
Asset-backed Securities [Member]
Less than 12 months
Fair value	55	48
Unrealized loss	(1)	0
12 months or more
Fair Value	3	3
Unrealized loss	0	0
Total
Fair value	58	51
Unrealized loss	(1)	0
Foreign government securities [Member]
Less than 12 months
Fair value	21	20
Unrealized loss	0	0
12 months or more
Fair Value	119	140
Unrealized loss	(13)	(17)
Total
Fair value	140	160
Unrealized loss	$ (13)	$ (17)