Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions		Sep. 30, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Less than 12 months
Fair value		$ 1,429	$ 797
Unrealized loss		(31)	(10)
12 months or more
Fair Value		250	393
Unrealized loss		(21)	(23)
Total
Fair value		1,679	1,190
Unrealized loss		$ (52)	$ (33)
Number of securities
Less than 12 months (in securities) | Security	[1]	342	125
12 months or more (in securities) | Security	[1]	37	82
Total (in securities) | Security	[1]	372	198
Number of securities with OTTI
Less than 12 months (in securities) | Security		5	3
12 months or more (in securities) | Security		4	7
Total (in securities) | Security		9	10
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value		$ 587	$ 64
Unrealized loss		(18)	0
12 months or more
Fair Value		4	25
Unrealized loss		0	(1)
Total
Fair value		591	89
Unrealized loss		(18)	(1)
US Treasury and Government [Member]
Less than 12 months
Fair value		31	139
Unrealized loss		0	0
12 months or more
Fair Value		0	68
Unrealized loss		0	(1)
Total
Fair value		31	207
Unrealized loss		0	(1)
Corporate securities [Member]
Less than 12 months
Fair value		373	189
Unrealized loss		(8)	(3)
12 months or more
Fair Value		99	104
Unrealized loss		(4)	(2)
Total
Fair value		472	293
Unrealized loss		(12)	(5)
RMBS [Member]
Less than 12 months
Fair value		315	205
Unrealized loss		(2)	(3)
12 months or more
Fair Value		91	159
Unrealized loss		(14)	(18)
Total
Fair value		406	364
Unrealized loss		(16)	(21)
CMBS [Member]
Less than 12 months
Fair value		24	36
Unrealized loss		0	0
12 months or more
Fair Value		2	19
Unrealized loss		0	0
Total
Fair value		26	55
Unrealized loss		0	0
Asset-backed Securities [Member]
Less than 12 months
Fair value		2	56
Unrealized loss		0	(2)
12 months or more
Fair Value		0	18
Unrealized loss		0	(1)
Total
Fair value		2	74
Unrealized loss		0	(3)
Foreign government securities [Member]
Less than 12 months
Fair value		97	108
Unrealized loss		(3)	(2)
12 months or more
Fair Value		54	0
Unrealized loss		(3)	0
Total
Fair value		151	108
Unrealized loss		$ (6)	$ (2)

[1] The number of securities does not add across because lots of the same securities have been purchased at different times and appear in both categories above (i.e. Less than 12 months and 12 months or more). If a security appears in both categories, it is counted only once in the total column.