Investments and Cash (Details 3) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Municipal Bonds [Member]
Investments
Fixed maturity securities	$ 5,047	$ 5,135
Available-for-sale Securities, Amortized Cost Basis	4,648	4,696
Municipal Bonds [Member] | Texas
Investments
Fixed maturity securities	762	775
Available-for-sale Securities, Amortized Cost Basis	701	708
Municipal Bonds [Member] | New York
Investments
Fixed maturity securities	664	673
Available-for-sale Securities, Amortized Cost Basis	618	620
Municipal Bonds [Member] | California
Investments
Fixed maturity securities	460	459
Available-for-sale Securities, Amortized Cost Basis	428	425
Municipal Bonds [Member] | Florida
Investments
Fixed maturity securities	360	356
Available-for-sale Securities, Amortized Cost Basis	325	319
Municipal Bonds [Member] | Illinois
Investments
Fixed maturity securities	281	287
Available-for-sale Securities, Amortized Cost Basis	257	260
Municipal Bonds [Member] | Massachusetts
Investments
Fixed maturity securities	222	225
Available-for-sale Securities, Amortized Cost Basis	199	199
Municipal Bonds [Member] | Washington
Investments
Fixed maturity securities	198	218
Available-for-sale Securities, Amortized Cost Basis	182	200
Municipal Bonds [Member] | Arizona
Investments
Fixed maturity securities	186	188
Available-for-sale Securities, Amortized Cost Basis	170	171
Municipal Bonds [Member] | GEORGIA
Investments
Fixed maturity securities		142
Available-for-sale Securities, Amortized Cost Basis		132
Municipal Bonds [Member] | MICHIGAN
Investments
Fixed maturity securities	139
Available-for-sale Securities, Amortized Cost Basis	127
Municipal Bonds [Member] | Pennsylvania
Investments
Fixed maturity securities	137	140
Available-for-sale Securities, Amortized Cost Basis	129	129
Municipal Bonds [Member] | All others
Investments
Fixed maturity securities	1,638	1,672
Available-for-sale Securities, Amortized Cost Basis	1,512	1,533
Pre Refunded Bond
Investments
Fixed maturity securities	554	496
State General Obligation
Investments
Fixed maturity securities	566	581
State General Obligation | Texas
Investments
Fixed maturity securities	87	88
State General Obligation | New York
Investments
Fixed maturity securities	12	22
State General Obligation | California
Investments
Fixed maturity securities	27	23
State General Obligation | Florida
Investments
Fixed maturity securities	47	47
State General Obligation | Illinois
Investments
Fixed maturity securities	14	15
State General Obligation | Massachusetts
Investments
Fixed maturity securities	41	42
State General Obligation | Washington
Investments
Fixed maturity securities	33	33
State General Obligation | Arizona
Investments
Fixed maturity securities	0	0
State General Obligation | GEORGIA
Investments
Fixed maturity securities		14
State General Obligation | MICHIGAN
Investments
Fixed maturity securities	0
State General Obligation | Pennsylvania
Investments
Fixed maturity securities	67	68
State General Obligation | All others
Investments
Fixed maturity securities	238	229
Local General Obligation
Investments
Fixed maturity securities	931	980
Local General Obligation | Texas
Investments
Fixed maturity securities	336	345
Local General Obligation | New York
Investments
Fixed maturity securities	66	58
Local General Obligation | California
Investments
Fixed maturity securities	73	77
Local General Obligation | Florida
Investments
Fixed maturity securities	49	50
Local General Obligation | Illinois
Investments
Fixed maturity securities	78	84
Local General Obligation | Massachusetts
Investments
Fixed maturity securities	18	18
Local General Obligation | Washington
Investments
Fixed maturity securities	21	40
Local General Obligation | Arizona
Investments
Fixed maturity securities	8	8
Local General Obligation | GEORGIA
Investments
Fixed maturity securities		20
Local General Obligation | MICHIGAN
Investments
Fixed maturity securities	29
Local General Obligation | Pennsylvania
Investments
Fixed maturity securities	31	32
Local General Obligation | All others
Investments
Fixed maturity securities	222	248
Revenue bonds
Investments
Fixed maturity securities	3,550	3,574
Available-for-sale Securities, Amortized Cost Basis	3,273	3,273
Revenue bonds | Tax Backed [Member]
Investments
Fixed maturity securities	712	720
Available-for-sale Securities, Amortized Cost Basis	654	656
Revenue bonds | Transportation [Member]
Investments
Fixed maturity securities	711	717
Available-for-sale Securities, Amortized Cost Basis	644	646
Revenue bonds | Municipal Utilities [Member]
Investments
Fixed maturity securities	562	567
Available-for-sale Securities, Amortized Cost Basis	516	519
Revenue bonds | Water and Sewer [Member]
Investments
Fixed maturity securities	549	567
Available-for-sale Securities, Amortized Cost Basis	508	520
Revenue bonds | Higher Education [Member]
Investments
Fixed maturity securities	440	430
Available-for-sale Securities, Amortized Cost Basis	403	389
Revenue bonds | Healthcare [Member]
Investments
Fixed maturity securities	328	323
Available-for-sale Securities, Amortized Cost Basis	302	296
Revenue bonds | All other revenue sources
Investments
Fixed maturity securities	248	250
Available-for-sale Securities, Amortized Cost Basis	246	247
Revenue bonds | Texas
Investments
Fixed maturity securities	339	342
Revenue bonds | New York
Investments
Fixed maturity securities	586	593
Revenue bonds | California
Investments
Fixed maturity securities	360	359
Revenue bonds | Florida
Investments
Fixed maturity securities	264	259
Revenue bonds | Illinois
Investments
Fixed maturity securities	189	188
Revenue bonds | Massachusetts
Investments
Fixed maturity securities	163	165
Revenue bonds | Washington
Investments
Fixed maturity securities	144	145
Revenue bonds | Arizona
Investments
Fixed maturity securities	178	180
Revenue bonds | GEORGIA
Investments
Fixed maturity securities		108
Revenue bonds | MICHIGAN
Investments
Fixed maturity securities	110
Revenue bonds | Pennsylvania
Investments
Fixed maturity securities	39	40
Revenue bonds | All others
Investments
Fixed maturity securities	$ 1,178	$ 1,195