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, 2013	Dec. 31, 2012
Municipal Bonds [Member]
Investments
Available-for-Sale Municipal Bonds, Fair Value	$ 4,554	$ 5,135
Available-for-Sale Municipal Bonds, Amortized Cost	4,411	4,696
Municipal Bonds [Member] | Texas
Investments
Available-for-Sale Municipal Bonds, Fair Value	653	775
Available-for-Sale Municipal Bonds, Amortized Cost	629	708
Municipal Bonds [Member] | New York
Investments
Available-for-Sale Municipal Bonds, Fair Value	589	673
Available-for-Sale Municipal Bonds, Amortized Cost	575	620
Municipal Bonds [Member] | California
Investments
Available-for-Sale Municipal Bonds, Fair Value	472	459
Available-for-Sale Municipal Bonds, Amortized Cost	452	425
Municipal Bonds [Member] | Florida
Investments
Available-for-Sale Municipal Bonds, Fair Value	334	356
Available-for-Sale Municipal Bonds, Amortized Cost	318	319
Municipal Bonds [Member] | Illinois
Investments
Available-for-Sale Municipal Bonds, Fair Value	240	287
Available-for-Sale Municipal Bonds, Amortized Cost	234	260
Municipal Bonds [Member] | Massachusetts
Investments
Available-for-Sale Municipal Bonds, Fair Value	207	225
Available-for-Sale Municipal Bonds, Amortized Cost	200	199
Municipal Bonds [Member] | Washington
Investments
Available-for-Sale Municipal Bonds, Fair Value	203	218
Available-for-Sale Municipal Bonds, Amortized Cost	199	200
Municipal Bonds [Member] | Arizona
Investments
Available-for-Sale Municipal Bonds, Fair Value	173	188
Available-for-Sale Municipal Bonds, Amortized Cost	170	171
Municipal Bonds [Member] | Georgia
Investments
Available-for-Sale Municipal Bonds, Fair Value	130	142
Available-for-Sale Municipal Bonds, Amortized Cost	125	132
Municipal Bonds [Member] | Pennsylvania
Investments
Available-for-Sale Municipal Bonds, Fair Value	128	140
Available-for-Sale Municipal Bonds, Amortized Cost	128	129
Municipal Bonds [Member] | All others
Investments
Available-for-Sale Municipal Bonds, Fair Value	1,425	1,672
Available-for-Sale Municipal Bonds, Amortized Cost	1,381	1,533
State General Obligation
Investments
Available-for-Sale Municipal Bonds, Fair Value	510	581
State General Obligation | Texas
Investments
Available-for-Sale Municipal Bonds, Fair Value	77	88
State General Obligation | New York
Investments
Available-for-Sale Municipal Bonds, Fair Value	12	22
State General Obligation | California
Investments
Available-for-Sale Municipal Bonds, Fair Value	32	23
State General Obligation | Florida
Investments
Available-for-Sale Municipal Bonds, Fair Value	33	47
State General Obligation | Illinois
Investments
Available-for-Sale Municipal Bonds, Fair Value	14	15
State General Obligation | Massachusetts
Investments
Available-for-Sale Municipal Bonds, Fair Value	44	42
State General Obligation | Washington
Investments
Available-for-Sale Municipal Bonds, Fair Value	31	33
State General Obligation | Arizona
Investments
Available-for-Sale Municipal Bonds, Fair Value	0	0
State General Obligation | Georgia
Investments
Available-for-Sale Municipal Bonds, Fair Value	0	14
State General Obligation | Pennsylvania
Investments
Available-for-Sale Municipal Bonds, Fair Value	13	68
State General Obligation | All others
Investments
Available-for-Sale Municipal Bonds, Fair Value	254	229
Local General Obligation
Investments
Available-for-Sale Municipal Bonds, Fair Value	888	980
Local General Obligation | Texas
Investments
Available-for-Sale Municipal Bonds, Fair Value	299	345
Local General Obligation | New York
Investments
Available-for-Sale Municipal Bonds, Fair Value	58	58
Local General Obligation | California
Investments
Available-for-Sale Municipal Bonds, Fair Value	86	77
Local General Obligation | Florida
Investments
Available-for-Sale Municipal Bonds, Fair Value	59	50
Local General Obligation | Illinois
Investments
Available-for-Sale Municipal Bonds, Fair Value	70	84
Local General Obligation | Massachusetts
Investments
Available-for-Sale Municipal Bonds, Fair Value	16	18
Local General Obligation | Washington
Investments
Available-for-Sale Municipal Bonds, Fair Value	19	40
Local General Obligation | Arizona
Investments
Available-for-Sale Municipal Bonds, Fair Value	7	8
Local General Obligation | Georgia
Investments
Available-for-Sale Municipal Bonds, Fair Value	28	20
Local General Obligation | Pennsylvania
Investments
Available-for-Sale Municipal Bonds, Fair Value	18	32
Local General Obligation | All others
Investments
Available-for-Sale Municipal Bonds, Fair Value	228	248
Revenue Bonds
Investments
Available-for-Sale Municipal Bonds, Fair Value	3,156	3,574
Available-for-Sale Municipal Bonds, Amortized Cost	3,062	3,273
Revenue Bonds | Tax backed
Investments
Available-for-Sale Municipal Bonds, Fair Value	708	720
Available-for-Sale Municipal Bonds, Amortized Cost	686	656
Revenue Bonds | Transportation
Investments
Available-for-Sale Municipal Bonds, Fair Value	642	717
Available-for-Sale Municipal Bonds, Amortized Cost	615	646
Revenue Bonds | Municipal utilities
Investments
Available-for-Sale Municipal Bonds, Fair Value	500	567
Available-for-Sale Municipal Bonds, Amortized Cost	482	519
Revenue Bonds | Water and sewer
Investments
Available-for-Sale Municipal Bonds, Fair Value	459	567
Available-for-Sale Municipal Bonds, Amortized Cost	453	520
Revenue Bonds | Higher education
Investments
Available-for-Sale Municipal Bonds, Fair Value	358	430
Available-for-Sale Municipal Bonds, Amortized Cost	353	389
Revenue Bonds | Healthcare
Investments
Available-for-Sale Municipal Bonds, Fair Value	289	323
Available-for-Sale Municipal Bonds, Amortized Cost	281	296
Revenue Bonds | All others
Investments
Available-for-Sale Municipal Bonds, Fair Value	200	250
Available-for-Sale Municipal Bonds, Amortized Cost	192	247
Revenue Bonds | Texas
Investments
Available-for-Sale Municipal Bonds, Fair Value	277	342
Revenue Bonds | New York
Investments
Available-for-Sale Municipal Bonds, Fair Value	519	593
Revenue Bonds | California
Investments
Available-for-Sale Municipal Bonds, Fair Value	354	359
Revenue Bonds | Florida
Investments
Available-for-Sale Municipal Bonds, Fair Value	242	259
Revenue Bonds | Illinois
Investments
Available-for-Sale Municipal Bonds, Fair Value	156	188
Revenue Bonds | Massachusetts
Investments
Available-for-Sale Municipal Bonds, Fair Value	147	165
Revenue Bonds | Washington
Investments
Available-for-Sale Municipal Bonds, Fair Value	153	145
Revenue Bonds | Arizona
Investments
Available-for-Sale Municipal Bonds, Fair Value	166	180
Revenue Bonds | Georgia
Investments
Available-for-Sale Municipal Bonds, Fair Value	102	108
Revenue Bonds | Pennsylvania
Investments
Available-for-Sale Municipal Bonds, Fair Value	97	40
Revenue Bonds | All others
Investments
Available-for-Sale Municipal Bonds, Fair Value	943	1,195
Pre-Refunded Bonds
Investments
Available-for-Sale Municipal Bonds, Fair Value	$ 525	$ 496