Investments and Cash - Fair Value of Available-for-Sale Municipal Bond Portfolio by State and Revenue Sources (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	$ 11,258		$ 10,615
Amortized Cost	10,739		10,392
State General Obligation [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	580	[1]	510	[1]
State General Obligation [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	60	[1]	77	[1]
State General Obligation [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	13	[1]	12	[1]
State General Obligation [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	45	[1]	32	[1]
State General Obligation [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	47	[1]	33	[1]
State General Obligation [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	20	[1]	14	[1]
State General Obligation [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	46	[1]	44	[1]
State General Obligation [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	67	[1]	31	[1]
State General Obligation [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	0	[1]	0	[1]
State General Obligation [Member] | Michigan [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	0	[1]	0	[1]
State General Obligation [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities			13	[1]
State General Obligation [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	6	[1]
State General Obligation [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	276	[1]	254	[1]
Local General Obligation [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	891	[1]	888	[1]
Local General Obligation [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	293	[1]	299	[1]
Local General Obligation [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	41	[1]	58	[1]
Local General Obligation [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	70	[1]	86	[1]
Local General Obligation [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	34	[1]	59	[1]
Local General Obligation [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	99	[1]	70	[1]
Local General Obligation [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	8	[1]	16	[1]
Local General Obligation [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	48	[1]	19	[1]
Local General Obligation [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	7	[1]	7	[1]
Local General Obligation [Member] | Michigan [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	0	[1]	28	[1]
Local General Obligation [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities			18	[1]
Local General Obligation [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	40	[1]
Local General Obligation [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	251	[1]	228	[1]
Revenue Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	3,478	[1]	3,156	[1]
Amortized Cost	3,235		3,062
Revenue Bonds [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	305	[1]	277	[1]
Revenue Bonds [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	551	[1]	519	[1]
Revenue Bonds [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	377	[1]	354	[1]
Revenue Bonds [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	256	[1]	242	[1]
Revenue Bonds [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	177	[1]	156	[1]
Revenue Bonds [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	169	[1]	147	[1]
Revenue Bonds [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	163	[1]	153	[1]
Revenue Bonds [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	170	[1]	166	[1]
Revenue Bonds [Member] | Michigan [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	132	[1]	102	[1]
Revenue Bonds [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities			97	[1]
Revenue Bonds [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	82	[1]
Revenue Bonds [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	1,096	[1]	943	[1]
Transportation [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	796		642
Amortized Cost	733		615
Water and Sewer [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	563		459
Amortized Cost	527		453
Tax backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	551		708
Amortized Cost	514		686
Higher education [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	527		358
Amortized Cost	492		353
Municipal utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	512		500
Amortized Cost	479		482
Healthcare [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	346		289
Amortized Cost	317		281
All Others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	183		200
Amortized Cost	173		192
Obligations of state and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	4,949	[1]	4,554	[1]
Amortized Cost	4,619	[1]	4,411	[1]
Fixed-maturity securities, available-for-sale, at fair value	846		525
Obligations of state and political subdivisions [Member] | Texas [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	658	[1]	653	[1]
Amortized Cost	613	[1]	629	[1]
Obligations of state and political subdivisions [Member] | New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	605	[1]	589	[1]
Amortized Cost	571	[1]	575	[1]
Obligations of state and political subdivisions [Member] | California [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	492	[1]	472	[1]
Amortized Cost	449	[1]	452	[1]
Obligations of state and political subdivisions [Member] | Florida [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	337	[1]	334	[1]
Amortized Cost	311	[1]	318	[1]
Obligations of state and political subdivisions [Member] | Illinois [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	296	[1]	240	[1]
Amortized Cost	275	[1]	234	[1]
Obligations of state and political subdivisions [Member] | Massachusetts [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	223	[1]	207	[1]
Amortized Cost	204	[1]	200	[1]
Obligations of state and political subdivisions [Member] | Washington [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	278	[1]	203	[1]
Amortized Cost	262	[1]	199	[1]
Obligations of state and political subdivisions [Member] | Arizona [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	177	[1]	173	[1]
Amortized Cost	165	[1]	170	[1]
Obligations of state and political subdivisions [Member] | Michigan [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	132	[1]	130	[1]
Amortized Cost	122	[1]	125	[1]
Obligations of state and political subdivisions [Member] | Georgia [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities			128	[1]
Amortized Cost			128	[1]
Obligations of state and political subdivisions [Member] | Ohio [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	128	[1]
Amortized Cost	119	[1]
Obligations of state and political subdivisions [Member] | All others [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed-maturity securities	1,623	[1]	1,425	[1]
Amortized Cost	$ 1,528	[1]	$ 1,381	[1]

[1] Excludes $846 million and $525 million as of December 31, 2014 and 2013, respectively, of pre-refunded bonds, at fair value. The credit ratings are based on the underlying ratings and do not include any benefit from bond insurance.