Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Less than 12 months
Fair value	$ 806	$ 2,325
Unrealized loss	(10)	(83)
12 months or more
Fair Value	975	332
Unrealized loss	(57)	(63)
Total
Fair value	1,781	2,657
Unrealized loss	$ (67)	$ (146)
Number of securities
Less than 12 months (in securities) | Security	244	622
12 months or more (in securities) | Security	264	60
Total (in securities) | Security	499	676
Number of securities with OTTI
Less than 12 months (in securities) | Security	17	8
12 months or more (in securities) | Security	15	9
Total (in securities) | Security	31	17
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 166	$ 1,110
Unrealized loss	(4)	(38)
12 months or more
Fair Value	281	6
Unrealized loss	(7)	(1)
Total
Fair value	447	1,116
Unrealized loss	(11)	(39)
US Treasury and Government [Member]
Less than 12 months
Fair value	151	87
Unrealized loss	0	(1)
12 months or more
Fair Value	18	0
Unrealized loss	(1)	0
Total
Fair value	169	87
Unrealized loss	(1)	(1)
Corporate securities [Member]
Less than 12 months
Fair value	201	492
Unrealized loss	(1)	(11)
12 months or more
Fair Value	240	118
Unrealized loss	(17)	(20)
Total
Fair value	441	610
Unrealized loss	(18)	(31)
Residential Mortgage-Backed Securities (RMBS) [Member]
Less than 12 months
Fair value	191	391
Unrealized loss	(5)	(23)
12 months or more
Fair Value	213	94
Unrealized loss	(12)	(15)
Total
Fair value	404	485
Unrealized loss	(17)	(38)
Commercial Mortgage Backed Securities (CMBS) [Member]
Less than 12 months
Fair value	29	165
Unrealized loss	0	(5)
12 months or more
Fair Value	80	0
Unrealized loss	(3)	0
Total
Fair value	109	165
Unrealized loss	(3)	(5)
Asset-backed Securities [Member]
Less than 12 months
Fair value	48	36
Unrealized loss	0	0
12 months or more
Fair Value	3	0
Unrealized loss	0	0
Total
Fair value	51	36
Unrealized loss	0	0
Foreign government securities [Member]
Less than 12 months
Fair value	20	44
Unrealized loss	0	(5)
12 months or more
Fair Value	140	114
Unrealized loss	(17)	(27)
Total
Fair value	160	158
Unrealized loss	$ (17)	$ (32)