Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions	Mar. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Less than 12 months
Fair value	$ 2,156	$ 2,325
Unrealized loss	(59)	(83)
12 months or more
Fair Value	335	332
Unrealized loss	(58)	(63)
Total
Fair value	2,491	2,657
Unrealized loss	$ (117)	$ (146)
Number of securities
Less than 12 months (in securities) | Security	542	622
12 months or more (in securities) | Security	62	60
Total (in securities) | Security	598	676
Number of securities with OTTI
Less than 12 months (in securities) | Security	12	8
12 months or more (in securities) | Security	9	9
Total (in securities) | Security	21	17
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value	$ 994	$ 1,110
Unrealized loss	(28)	(38)
12 months or more
Fair Value	6	6
Unrealized loss	(1)	(1)
Total
Fair value	1,000	1,116
Unrealized loss	(29)	(39)
US government and agencies [Member]
Less than 12 months
Fair value	172	87
Unrealized loss	(1)	(1)
12 months or more
Fair Value	0	0
Unrealized loss	0	0
Total
Fair value	172	87
Unrealized loss	(1)	(1)
Corporate securities [Member]
Less than 12 months
Fair value	356	492
Unrealized loss	(8)	(11)
12 months or more
Fair Value	120	118
Unrealized loss	(19)	(20)
Total
Fair value	476	610
Unrealized loss	(27)	(31)
RMBS [Member]
Less than 12 months
Fair value	377	391
Unrealized loss	(13)	(23)
12 months or more
Fair Value	93	94
Unrealized loss	(13)	(15)
Total
Fair value	470	485
Unrealized loss	(26)	(38)
CMBS [Member]
Less than 12 months
Fair value	160	165
Unrealized loss	(5)	(5)
12 months or more
Fair Value	1	0
Unrealized loss	0	0
Total
Fair value	161	165
Unrealized loss	(5)	(5)
Asset-backed Securities [Member]
Less than 12 months
Fair value	52	36
Unrealized loss	0	0
12 months or more
Fair Value	0	0
Unrealized loss	0	0
Total
Fair value	52	36
Unrealized loss	0	0
Foreign government securities [Member]
Less than 12 months
Fair value	45	44
Unrealized loss	(4)	(5)
12 months or more
Fair Value	115	114
Unrealized loss	(25)	(27)
Total
Fair value	160	158
Unrealized loss	$ (29)	$ (32)