Investments and Cash - Gross Unrealized Loss by Length of Time (Details) $ in Millions		Dec. 31, 2016 USD ($) Security	Dec. 31, 2015 USD ($) Security
Less than 12 months
Fair value		$ 2,325	$ 1,966
Unrealized loss		(83)	(42)
12 months or more
Fair Value		332	276
Unrealized loss		(63)	(36)
Total
Fair value		2,657	2,242
Unrealized loss		$ (146)	$ (78)
Number of securities
Less than 12 months (in securities) | Security	[1]	622	335
12 months or more (in securities) | Security	[1]	60	71
Total (in securities) | Security	[1]	676	396
Number of securities with OTTI
Less than 12 months (in securities) | Security		8	9
12 months or more (in securities) | Security		9	4
Total (in securities) | Security		17	13
Obligations of state and political subdivisions [Member]
Less than 12 months
Fair value		$ 1,110	$ 316
Unrealized loss		(38)	(10)
12 months or more
Fair Value		6	7
Unrealized loss		(1)	0
Total
Fair value		1,116	323
Unrealized loss		(39)	(10)
US Treasury and Government [Member]
Less than 12 months
Fair value		87	77
Unrealized loss		(1)	0
12 months or more
Fair Value		0	0
Unrealized loss		0	0
Total
Fair value		87	77
Unrealized loss		(1)	0
Corporate securities [Member]
Less than 12 months
Fair value		492	381
Unrealized loss		(11)	(8)
12 months or more
Fair Value		118	95
Unrealized loss		(20)	(15)
Total
Fair value		610	476
Unrealized loss		(31)	(23)
Residential Mortgage-Backed Securities (RMBS) [Member]
Less than 12 months
Fair value		391	438
Unrealized loss		(23)	(8)
12 months or more
Fair Value		94	90
Unrealized loss		(15)	(14)
Total
Fair value		485	528
Unrealized loss		(38)	(22)
Commercial Mortgage Backed Securities (CMBS) [Member]
Less than 12 months
Fair value		165	140
Unrealized loss		(5)	(2)
12 months or more
Fair Value		0	2
Unrealized loss		0	0
Total
Fair value		165	142
Unrealized loss		(5)	(2)
Asset-backed Securities [Member]
Less than 12 months
Fair value		36	517
Unrealized loss		0	(10)
12 months or more
Fair Value		0	0
Unrealized loss		0	0
Total
Fair value		36	517
Unrealized loss		0	(10)
Foreign government securities [Member]
Less than 12 months
Fair value		44	97
Unrealized loss		(5)	(4)
12 months or more
Fair Value		114	82
Unrealized loss		(27)	(7)
Total
Fair value		158	179
Unrealized loss		$ (32)	$ (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.