Investments and Cash (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Municipal Bonds [Member]
Investments
Fixed maturity securities	$ 5,135	$ 5,052
Available-for-sale Securities, Amortized Cost Basis	4,696	4,720
Municipal Bonds [Member] | Texas
Investments
Fixed maturity securities	775	774
Available-for-sale Securities, Amortized Cost Basis	708	724
Municipal Bonds [Member] | New York
Investments
Fixed maturity securities	673	695
Available-for-sale Securities, Amortized Cost Basis	620	654
Municipal Bonds [Member] | California
Investments
Fixed maturity securities	459	367
Available-for-sale Securities, Amortized Cost Basis	425	336
Municipal Bonds [Member] | Florida
Investments
Fixed maturity securities	356	343
Available-for-sale Securities, Amortized Cost Basis	319	317
Municipal Bonds [Member] | Illinois
Investments
Fixed maturity securities	287	300
Available-for-sale Securities, Amortized Cost Basis	260	281
Municipal Bonds [Member] | Massachusetts
Investments
Fixed maturity securities	225	216
Available-for-sale Securities, Amortized Cost Basis	199	199
Municipal Bonds [Member] | Washington
Investments
Fixed maturity securities	218	214
Available-for-sale Securities, Amortized Cost Basis	200	200
Municipal Bonds [Member] | Arizona
Investments
Fixed maturity securities	188	172
Available-for-sale Securities, Amortized Cost Basis	171	163
Municipal Bonds [Member] | Georgia
Investments
Fixed maturity securities	142
Available-for-sale Securities, Amortized Cost Basis	132
Municipal Bonds [Member] | Pennsylvania
Investments
Fixed maturity securities	140
Available-for-sale Securities, Amortized Cost Basis	129
Municipal Bonds [Member] | Ohio
Investments
Fixed maturity securities		139
Available-for-sale Securities, Amortized Cost Basis		129
Municipal Bonds [Member] | Michigan
Investments
Fixed maturity securities		136
Available-for-sale Securities, Amortized Cost Basis		129
Municipal Bonds [Member] | All others
Investments
Fixed maturity securities	1,672	1,696
Available-for-sale Securities, Amortized Cost Basis	1,533	1,588
Pre Refunded Bond
Investments
Fixed maturity securities	496	403
State General Obligation
Investments
Fixed maturity securities	581	559
State General Obligation | Texas
Investments
Fixed maturity securities	88	86
State General Obligation | New York
Investments
Fixed maturity securities	22	12
State General Obligation | California
Investments
Fixed maturity securities	23	19
State General Obligation | Florida
Investments
Fixed maturity securities	47	34
State General Obligation | Illinois
Investments
Fixed maturity securities	15	16
State General Obligation | Massachusetts
Investments
Fixed maturity securities	42	43
State General Obligation | Washington
Investments
Fixed maturity securities	33	38
State General Obligation | Arizona
Investments
Fixed maturity securities	0	0
State General Obligation | Georgia
Investments
Fixed maturity securities	14
State General Obligation | Pennsylvania
Investments
Fixed maturity securities	68
State General Obligation | Ohio
Investments
Fixed maturity securities		0
State General Obligation | Michigan
Investments
Fixed maturity securities		0
State General Obligation | All others
Investments
Fixed maturity securities	229	311
Local General Obligation
Investments
Fixed maturity securities	980	1,033
Local General Obligation | Texas
Investments
Fixed maturity securities	345	342
Local General Obligation | New York
Investments
Fixed maturity securities	58	60
Local General Obligation | California
Investments
Fixed maturity securities	77	51
Local General Obligation | Florida
Investments
Fixed maturity securities	50	62
Local General Obligation | Illinois
Investments
Fixed maturity securities	84	87
Local General Obligation | Massachusetts
Investments
Fixed maturity securities	18	9
Local General Obligation | Washington
Investments
Fixed maturity securities	40	53
Local General Obligation | Arizona
Investments
Fixed maturity securities	8	8
Local General Obligation | Georgia
Investments
Fixed maturity securities	20
Local General Obligation | Pennsylvania
Investments
Fixed maturity securities	32
Local General Obligation | Ohio
Investments
Fixed maturity securities		53
Local General Obligation | Michigan
Investments
Fixed maturity securities		37
Local General Obligation | All others
Investments
Fixed maturity securities	248	271
Revenue bonds
Investments
Fixed maturity securities	3,574	3,460
Available-for-sale Securities, Amortized Cost Basis	3,273	3,243
Revenue bonds | Tax Backed [Member]
Investments
Fixed maturity securities	720	717
Available-for-sale Securities, Amortized Cost Basis	656	670
Revenue bonds | Transportation [Member]
Investments
Fixed maturity securities	717	800
Available-for-sale Securities, Amortized Cost Basis	646	749
Revenue bonds | Water and sewer
Investments
Fixed maturity securities	567	530
Available-for-sale Securities, Amortized Cost Basis	520	501
Revenue bonds | Municipal Utilities [Member]
Investments
Fixed maturity securities	567	529
Available-for-sale Securities, Amortized Cost Basis	519	494
Revenue bonds | Higher Education [Member]
Investments
Fixed maturity securities	430	332
Available-for-sale Securities, Amortized Cost Basis	389	307
Revenue bonds | Healthcare [Member]
Investments
Fixed maturity securities	323	273
Available-for-sale Securities, Amortized Cost Basis	296	258
Revenue bonds | All other revenue sources
Investments
Fixed maturity securities	250	279
Available-for-sale Securities, Amortized Cost Basis	247	264
Revenue bonds | Texas
Investments
Fixed maturity securities	342	346
Revenue bonds | New York
Investments
Fixed maturity securities	593	623
Revenue bonds | California
Investments
Fixed maturity securities	359	297
Revenue bonds | Florida
Investments
Fixed maturity securities	259	247
Revenue bonds | Illinois
Investments
Fixed maturity securities	188	197
Revenue bonds | Massachusetts
Investments
Fixed maturity securities	165	164
Revenue bonds | Washington
Investments
Fixed maturity securities	145	123
Revenue bonds | Arizona
Investments
Fixed maturity securities	180	164
Revenue bonds | Georgia
Investments
Fixed maturity securities	108
Revenue bonds | Pennsylvania
Investments
Fixed maturity securities	40
Revenue bonds | Ohio
Investments
Fixed maturity securities		86
Revenue bonds | Michigan
Investments
Fixed maturity securities		99
Revenue bonds | All others
Investments
Fixed maturity securities	$ 1,195	$ 1,114