Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions		Dec. 31, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Less than 12 months
Fair value		$ 1,966	$ 797
Unrealized loss		(42)	(10)
12 months or more
Fair Value		276	393
Unrealized loss		(36)	(23)
Total
Fair value		2,242	1,190
Unrealized loss		$ (78)	$ (33)
Number of securities
Less than 12 months (in securities) | Security	[1]	335	125
12 months or more (in securities) | Security	[1]	71	82
Total (in securities) | Security	[1]	396	198
Number of securities with OTTI
Less than 12 months (in securities) | Security		9	3
12 months or more (in securities) | Security		4	7
Total (in securities) | Security		13	10
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value		$ 316	$ 64
Unrealized loss		(10)	0
12 months or more
Fair Value		7	25
Unrealized loss		0	(1)
Total
Fair value		323	89
Unrealized loss		(10)	(1)
US Treasury and Government [Member]
Less than 12 months
Fair value		77	139
Unrealized loss		0	0
12 months or more
Fair Value		0	68
Unrealized loss		0	(1)
Total
Fair value		77	207
Unrealized loss		0	(1)
Corporate securities [Member]
Less than 12 months
Fair value		381	189
Unrealized loss		(8)	(3)
12 months or more
Fair Value		95	104
Unrealized loss		(15)	(2)
Total
Fair value		476	293
Unrealized loss		(23)	(5)
Residential Mortgage-Backed Securities (RMBS) [Member]
Less than 12 months
Fair value		438	205
Unrealized loss		(8)	(3)
12 months or more
Fair Value		90	159
Unrealized loss		(14)	(18)
Total
Fair value		528	364
Unrealized loss		(22)	(21)
Commercial Mortgage Backed Securities (CMBS) [Member]
Less than 12 months
Fair value		140	36
Unrealized loss		(2)	0
12 months or more
Fair Value		2	19
Unrealized loss		0	0
Total
Fair value		142	55
Unrealized loss		(2)	0
Asset-backed Securities [Member]
Less than 12 months
Fair value		517	56
Unrealized loss		(10)	(2)
12 months or more
Fair Value		0	18
Unrealized loss		0	(1)
Total
Fair value		517	74
Unrealized loss		(10)	(3)
Foreign government securities [Member]
Less than 12 months
Fair value		97	108
Unrealized loss		(4)	(2)
12 months or more
Fair Value		82	0
Unrealized loss		(7)	0
Total
Fair value		179	108
Unrealized loss		$ (11)	$ (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.