Investments (Details 3) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Municipal Bonds [Member]
Investments
Available-for-sale Securities, Fair Value Disclosure	$ 5,155	$ 5,052
Available-for-sale Securities, Amortized Cost Basis	4,697	4,720
Municipal Bonds [Member] | Texas
Investments
Available-for-sale Securities, Fair Value Disclosure	799	774
Available-for-sale Securities, Amortized Cost Basis	728	724
Municipal Bonds [Member] | New York
Investments
Available-for-sale Securities, Fair Value Disclosure	687	695
Available-for-sale Securities, Amortized Cost Basis	631	654
Municipal Bonds [Member] | California
Investments
Available-for-sale Securities, Fair Value Disclosure	419	367
Available-for-sale Securities, Amortized Cost Basis	374	336
Municipal Bonds [Member] | Florida
Investments
Available-for-sale Securities, Fair Value Disclosure	357	343
Available-for-sale Securities, Amortized Cost Basis	320	317
Municipal Bonds [Member] | Illinois
Investments
Available-for-sale Securities, Fair Value Disclosure	290	300
Available-for-sale Securities, Amortized Cost Basis	265	281
Municipal Bonds [Member] | WASHINGTON
Investments
Available-for-sale Securities, Fair Value Disclosure	227	214
Available-for-sale Securities, Amortized Cost Basis	207	200
Municipal Bonds [Member] | Massachusetts
Investments
Available-for-sale Securities, Fair Value Disclosure	225	216
Available-for-sale Securities, Amortized Cost Basis	200	199
Municipal Bonds [Member] | Arizona
Investments
Available-for-sale Securities, Fair Value Disclosure	179	172
Available-for-sale Securities, Amortized Cost Basis	164	163
Municipal Bonds [Member] | Ohio
Investments
Available-for-sale Securities, Fair Value Disclosure		139
Available-for-sale Securities, Amortized Cost Basis		129
Municipal Bonds [Member] | Georgia
Investments
Available-for-sale Securities, Fair Value Disclosure	144
Available-for-sale Securities, Amortized Cost Basis	133
Municipal Bonds [Member] | Michigan
Investments
Available-for-sale Securities, Fair Value Disclosure	143	136
Available-for-sale Securities, Amortized Cost Basis	131	129
Municipal Bonds [Member] | All others
Investments
Available-for-sale Securities, Fair Value Disclosure	1,685	1,696
Available-for-sale Securities, Amortized Cost Basis	1,544	1,588
Pre Refunded Bond [Member]
Investments
Available-for-sale Securities, Fair Value Disclosure	559	403
State General Obligation
Investments
Available-for-sale Securities, Fair Value Disclosure	639	559
State General Obligation | Texas
Investments
Available-for-sale Securities, Fair Value Disclosure	89	86
State General Obligation | New York
Investments
Available-for-sale Securities, Fair Value Disclosure	13	12
State General Obligation | California
Investments
Available-for-sale Securities, Fair Value Disclosure	23	19
State General Obligation | Florida
Investments
Available-for-sale Securities, Fair Value Disclosure	98	34
State General Obligation | Illinois
Investments
Available-for-sale Securities, Fair Value Disclosure	14	16
State General Obligation | WASHINGTON
Investments
Available-for-sale Securities, Fair Value Disclosure	34	38
State General Obligation | Massachusetts
Investments
Available-for-sale Securities, Fair Value Disclosure	42	43
State General Obligation | Arizona
Investments
Available-for-sale Securities, Fair Value Disclosure	0	0
State General Obligation | Ohio
Investments
Available-for-sale Securities, Fair Value Disclosure		0
State General Obligation | Georgia
Investments
Available-for-sale Securities, Fair Value Disclosure	14
State General Obligation | Michigan
Investments
Available-for-sale Securities, Fair Value Disclosure	0	0
State General Obligation | All others
Investments
Available-for-sale Securities, Fair Value Disclosure	312	311
Local General Obligation
Investments
Available-for-sale Securities, Fair Value Disclosure	934	1,033
Local General Obligation | Texas
Investments
Available-for-sale Securities, Fair Value Disclosure	353	342
Local General Obligation | New York
Investments
Available-for-sale Securities, Fair Value Disclosure	67	60
Local General Obligation | California
Investments
Available-for-sale Securities, Fair Value Disclosure	71	51
Local General Obligation | Florida
Investments
Available-for-sale Securities, Fair Value Disclosure	0	62
Local General Obligation | Illinois
Investments
Available-for-sale Securities, Fair Value Disclosure	83	87
Local General Obligation | WASHINGTON
Investments
Available-for-sale Securities, Fair Value Disclosure	40	53
Local General Obligation | Massachusetts
Investments
Available-for-sale Securities, Fair Value Disclosure	10	9
Local General Obligation | Arizona
Investments
Available-for-sale Securities, Fair Value Disclosure	8	8
Local General Obligation | Ohio
Investments
Available-for-sale Securities, Fair Value Disclosure		53
Local General Obligation | Georgia
Investments
Available-for-sale Securities, Fair Value Disclosure	20
Local General Obligation | Michigan
Investments
Available-for-sale Securities, Fair Value Disclosure	29	37
Local General Obligation | All others
Investments
Available-for-sale Securities, Fair Value Disclosure	253	271
Revenue bonds
Investments
Available-for-sale Securities, Fair Value Disclosure	3,582	3,460
Available-for-sale Securities, Amortized Cost Basis	3,266	3,243
Revenue bonds | Transportation
Investments
Available-for-sale Securities, Fair Value Disclosure	746	717
Available-for-sale Securities, Amortized Cost Basis	679	670
Revenue bonds | Tax Backed
Investments
Available-for-sale Securities, Fair Value Disclosure	730	800
Available-for-sale Securities, Amortized Cost Basis	660	749
Revenue bonds | Municipal utilities
Investments
Available-for-sale Securities, Fair Value Disclosure	570	530
Available-for-sale Securities, Amortized Cost Basis	522	501
Revenue bonds | Water and sewer
Investments
Available-for-sale Securities, Fair Value Disclosure	565	529
Available-for-sale Securities, Amortized Cost Basis	516	494
Revenue bonds | Higher Education
Investments
Available-for-sale Securities, Fair Value Disclosure	422	332
Available-for-sale Securities, Amortized Cost Basis	381	307
Revenue bonds | Healthcare
Investments
Available-for-sale Securities, Fair Value Disclosure	313	273
Available-for-sale Securities, Amortized Cost Basis	287	258
Revenue bonds | All other revenue sources
Investments
Available-for-sale Securities, Fair Value Disclosure	236	279
Available-for-sale Securities, Amortized Cost Basis	221	264
Revenue bonds | Texas
Investments
Available-for-sale Securities, Fair Value Disclosure	357	346
Revenue bonds | New York
Investments
Available-for-sale Securities, Fair Value Disclosure	607	623
Revenue bonds | California
Investments
Available-for-sale Securities, Fair Value Disclosure	325	297
Revenue bonds | Florida
Investments
Available-for-sale Securities, Fair Value Disclosure	259	247
Revenue bonds | Illinois
Investments
Available-for-sale Securities, Fair Value Disclosure	193	197
Revenue bonds | WASHINGTON
Investments
Available-for-sale Securities, Fair Value Disclosure	153	123
Revenue bonds | Massachusetts
Investments
Available-for-sale Securities, Fair Value Disclosure	173	164
Revenue bonds | Arizona
Investments
Available-for-sale Securities, Fair Value Disclosure	171	164
Revenue bonds | Ohio
Investments
Available-for-sale Securities, Fair Value Disclosure		86
Revenue bonds | Georgia
Investments
Available-for-sale Securities, Fair Value Disclosure	110
Revenue bonds | Michigan
Investments
Available-for-sale Securities, Fair Value Disclosure	114	99
Revenue bonds | All others
Investments
Available-for-sale Securities, Fair Value Disclosure	$ 1,120	$ 1,114