Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Jun. 30, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Less than 12 months
Fair value	$ 1,516	$ 2,325
Unrealized loss	(35)	(83)
12 months or more
Fair Value	392	332
Unrealized loss	(49)	(63)
Total
Fair value	1,908	2,657
Unrealized loss	$ (84)	$ (146)
Number of securities
Less than 12 months (in securities) | Security	407	622
12 months or more (in securities) | Security	84	60
Total (in securities) | Security	484	676
Number of securities with OTTI
Less than 12 months (in securities) | Security	10	8
12 months or more (in securities) | Security	10	9
Total (in securities) | Security	20	17
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 633	$ 1,110
Unrealized loss	(18)	(38)
12 months or more
Fair Value	7	6
Unrealized loss	(1)	(1)
Total
Fair value	640	1,116
Unrealized loss	(19)	(39)
US government and agencies [Member]
Less than 12 months
Fair value	124	87
Unrealized loss	0	(1)
12 months or more
Fair Value	4	0
Unrealized loss	0	0
Total
Fair value	128	87
Unrealized loss	0	(1)
Corporate securities [Member]
Less than 12 months
Fair value	281	492
Unrealized loss	(5)	(11)
12 months or more
Fair Value	137	118
Unrealized loss	(18)	(20)
Total
Fair value	418	610
Unrealized loss	(23)	(31)
RMBS [Member]
Less than 12 months
Fair value	314	391
Unrealized loss	(6)	(23)
12 months or more
Fair Value	98	94
Unrealized loss	(7)	(15)
Total
Fair value	412	485
Unrealized loss	(13)	(38)
CMBS [Member]
Less than 12 months
Fair value	98	165
Unrealized loss	(4)	(5)
12 months or more
Fair Value	17	0
Unrealized loss	0	0
Total
Fair value	115	165
Unrealized loss	(4)	(5)
Asset-backed Securities [Member]
Less than 12 months
Fair value	20	36
Unrealized loss	0	0
12 months or more
Fair Value	0	0
Unrealized loss	0	0
Total
Fair value	20	36
Unrealized loss	0	0
Foreign government securities [Member]
Less than 12 months
Fair value	46	44
Unrealized loss	(2)	(5)
12 months or more
Fair Value	129	114
Unrealized loss	(23)	(27)
Total
Fair value	175	158
Unrealized loss	$ (25)	$ (32)