Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Sep. 30, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Less than 12 months
Fair value	$ 953	$ 2,325
Unrealized loss	(13)	(83)
12 months or more
Fair Value	905	332
Unrealized loss	(57)	(63)
Total
Fair value	1,858	2,657
Unrealized loss	$ (70)	$ (146)
Number of securities
Less than 12 months (in securities) | Security	292	622
12 months or more (in securities) | Security	230	60
Total (in securities) | Security	513	676
Number of securities with OTTI
Less than 12 months (in securities) | Security	9	8
12 months or more (in securities) | Security	14	9
Total (in securities) | Security	23	17
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 445	$ 1,110
Unrealized loss	(10)	(38)
12 months or more
Fair Value	253	6
Unrealized loss	(7)	(1)
Total
Fair value	698	1,116
Unrealized loss	(17)	(39)
US government and agencies [Member]
Less than 12 months
Fair value	114	87
Unrealized loss	0	(1)
12 months or more
Fair Value	4	0
Unrealized loss	0	0
Total
Fair value	118	87
Unrealized loss	0	(1)
Corporate securities [Member]
Less than 12 months
Fair value	136	492
Unrealized loss	(1)	(11)
12 months or more
Fair Value	252	118
Unrealized loss	(17)	(20)
Total
Fair value	388	610
Unrealized loss	(18)	(31)
RMBS [Member]
Less than 12 months
Fair value	107	391
Unrealized loss	(1)	(23)
12 months or more
Fair Value	170	94
Unrealized loss	(11)	(15)
Total
Fair value	277	485
Unrealized loss	(12)	(38)
CMBS [Member]
Less than 12 months
Fair value	50	165
Unrealized loss	0	(5)
12 months or more
Fair Value	76	0
Unrealized loss	(4)	0
Total
Fair value	126	165
Unrealized loss	(4)	(5)
Asset-backed Securities [Member]
Less than 12 months
Fair value	66	36
Unrealized loss	0	0
12 months or more
Fair Value	3	0
Unrealized loss	0	0
Total
Fair value	69	36
Unrealized loss	0	0
Foreign government securities [Member]
Less than 12 months
Fair value	35	44
Unrealized loss	(1)	(5)
12 months or more
Fair Value	147	114
Unrealized loss	(18)	(27)
Total
Fair value	182	158
Unrealized loss	$ (19)	$ (32)