Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions		Jun. 30, 2016 USD ($) Security		Dec. 31, 2015 USD ($) Security
Less than 12 months
Fair value		$ 770		$ 1,966
Unrealized loss		(36)		(42)
12 months or more
Fair Value		258		276
Unrealized loss		(57)		(36)
Total
Fair value		1,028		2,242
Unrealized loss		$ (93)		$ (78)
Number of securities
Less than 12 months (in securities) | Security	[1]	68		335
12 months or more (in securities) | Security	[1]	53		71
Total (in securities) | Security	[1]	113		396
Number of securities with OTTI
Less than 12 months (in securities) | Security		8	[1]	9
12 months or more (in securities) | Security		7	[1]	4
Total (in securities) | Security		14	[1]	13
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value		$ 25		$ 316
Unrealized loss		0		(10)
12 months or more
Fair Value		10		7
Unrealized loss		(1)		0
Total
Fair value		35		323
Unrealized loss		(1)		(10)
US government and agencies [Member]
Less than 12 months
Fair value		10		77
Unrealized loss		0		0
12 months or more
Fair Value		0		0
Unrealized loss		0		0
Total
Fair value		10		77
Unrealized loss		0		0
Corporate securities [Member]
Less than 12 months
Fair value		54		381
Unrealized loss		(5)		(8)
12 months or more
Fair Value		81		95
Unrealized loss		(33)		(15)
Total
Fair value		135		476
Unrealized loss		(38)		(23)
RMBS [Member]
Less than 12 months
Fair value		47		438
Unrealized loss		(5)		(8)
12 months or more
Fair Value		115		90
Unrealized loss		(14)		(14)
Total
Fair value		162		528
Unrealized loss		(19)		(22)
CMBS [Member]
Less than 12 months
Fair value		7		140
Unrealized loss		0		(2)
12 months or more
Fair Value		0		2
Unrealized loss		0		0
Total
Fair value		7		142
Unrealized loss		0		(2)
Asset-backed Securities [Member]
Less than 12 months
Fair value		543		517
Unrealized loss		(17)		(10)
12 months or more
Fair Value		0		0
Unrealized loss		0		0
Total
Fair value		543		517
Unrealized loss		(17)		(10)
Foreign government securities [Member]
Less than 12 months
Fair value		84		97
Unrealized loss		(9)		(4)
12 months or more
Fair Value		52		82
Unrealized loss		(9)		(7)
Total
Fair value		136		179
Unrealized loss		$ (18)		$ (11)

[1] The number of securities does not add across because lots consisting 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.