Investments (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 manager	Dec. 31, 2011 Municipal Bonds	Dec. 31, 2010 Municipal Bonds	Dec. 31, 2011 Municipal Bonds Texas	Dec. 31, 2010 Municipal Bonds Texas	Dec. 31, 2011 Municipal Bonds New York	Dec. 31, 2010 Municipal Bonds New York	Dec. 31, 2011 Municipal Bonds California	Dec. 31, 2010 Municipal Bonds California	Dec. 31, 2011 Municipal Bonds Florida	Dec. 31, 2010 Municipal Bonds Florida	Dec. 31, 2011 Municipal Bonds Illinois	Dec. 31, 2010 Municipal Bonds Illinois	Dec. 31, 2011 Municipal Bonds Massachusetts	Dec. 31, 2010 Municipal Bonds Massachusetts	Dec. 31, 2011 Municipal Bonds Washington	Dec. 31, 2010 Municipal Bonds Washington	Dec. 31, 2011 Municipal Bonds Arizona	Dec. 31, 2010 Municipal Bonds Arizona	Dec. 31, 2011 Municipal Bonds Ohio	Dec. 31, 2011 Municipal Bonds Michigan	Dec. 31, 2010 Municipal Bonds Michigan	Dec. 31, 2010 Municipal Bonds Georgia	Dec. 31, 2011 Municipal Bonds All other states	Dec. 31, 2010 Municipal Bonds All other states	Dec. 31, 2011 Pre-refunded Bond	Dec. 31, 2010 Pre-refunded Bond	Dec. 31, 2011 State General Obligation	Dec. 31, 2010 State General Obligation	Dec. 31, 2011 State General Obligation Texas	Dec. 31, 2010 State General Obligation Texas	Dec. 31, 2011 State General Obligation New York	Dec. 31, 2010 State General Obligation New York	Dec. 31, 2011 State General Obligation California	Dec. 31, 2010 State General Obligation California	Dec. 31, 2011 State General Obligation Florida	Dec. 31, 2010 State General Obligation Florida	Dec. 31, 2011 State General Obligation Illinois	Dec. 31, 2010 State General Obligation Illinois	Dec. 31, 2011 State General Obligation Massachusetts	Dec. 31, 2010 State General Obligation Massachusetts	Dec. 31, 2011 State General Obligation Washington	Dec. 31, 2010 State General Obligation Washington	Dec. 31, 2010 State General Obligation Georgia	Dec. 31, 2011 State General Obligation All other states	Dec. 31, 2010 State General Obligation All other states	Dec. 31, 2011 Local General Obligation	Dec. 31, 2010 Local General Obligation	Dec. 31, 2011 Local General Obligation Texas	Dec. 31, 2010 Local General Obligation Texas	Dec. 31, 2011 Local General Obligation New York	Dec. 31, 2010 Local General Obligation New York	Dec. 31, 2011 Local General Obligation California	Dec. 31, 2010 Local General Obligation California	Dec. 31, 2011 Local General Obligation Florida	Dec. 31, 2010 Local General Obligation Florida	Dec. 31, 2011 Local General Obligation Illinois	Dec. 31, 2010 Local General Obligation Illinois	Dec. 31, 2011 Local General Obligation Massachusetts	Dec. 31, 2010 Local General Obligation Massachusetts	Dec. 31, 2011 Local General Obligation Washington	Dec. 31, 2010 Local General Obligation Washington	Dec. 31, 2011 Local General Obligation Arizona	Dec. 31, 2010 Local General Obligation Arizona	Dec. 31, 2011 Local General Obligation Ohio	Dec. 31, 2011 Local General Obligation Michigan	Dec. 31, 2010 Local General Obligation Michigan	Dec. 31, 2010 Local General Obligation Georgia	Dec. 31, 2011 Local General Obligation All other states	Dec. 31, 2010 Local General Obligation All other states	Dec. 31, 2011 Revenue bonds	Dec. 31, 2010 Revenue bonds	Dec. 31, 2011 Revenue bonds Tax Backed	Dec. 31, 2010 Revenue bonds Tax Backed	Dec. 31, 2011 Revenue bonds Transportation	Dec. 31, 2010 Revenue bonds Transportation	Dec. 31, 2011 Revenue bonds Water and sewer	Dec. 31, 2010 Revenue bonds Water and sewer	Dec. 31, 2011 Revenue bonds Municipal utilities	Dec. 31, 2010 Revenue bonds Municipal utilities	Dec. 31, 2011 Revenue bonds Higher Education	Dec. 31, 2010 Revenue bonds Higher Education	Dec. 31, 2011 Revenue bonds Healthcare	Dec. 31, 2010 Revenue bonds Healthcare	Dec. 31, 2011 Revenue bonds All other revenue sources	Dec. 31, 2010 Revenue bonds All other revenue sources	Dec. 31, 2011 Revenue bonds Texas	Dec. 31, 2010 Revenue bonds Texas	Dec. 31, 2011 Revenue bonds New York	Dec. 31, 2010 Revenue bonds New York	Dec. 31, 2011 Revenue bonds California	Dec. 31, 2010 Revenue bonds California	Dec. 31, 2011 Revenue bonds Florida	Dec. 31, 2010 Revenue bonds Florida	Dec. 31, 2011 Revenue bonds Illinois	Dec. 31, 2010 Revenue bonds Illinois	Dec. 31, 2011 Revenue bonds Massachusetts	Dec. 31, 2010 Revenue bonds Massachusetts	Dec. 31, 2011 Revenue bonds Washington	Dec. 31, 2010 Revenue bonds Washington	Dec. 31, 2011 Revenue bonds Arizona	Dec. 31, 2010 Revenue bonds Arizona	Dec. 31, 2011 Revenue bonds Ohio	Dec. 31, 2011 Revenue bonds Michigan	Dec. 31, 2010 Revenue bonds Michigan	Dec. 31, 2010 Revenue bonds Georgia	Dec. 31, 2011 Revenue bonds All other states	Dec. 31, 2010 Revenue bonds All other states
Investments
Fair Value		$ 5,052.0	$ 4,481.6	$ 773.9	$ 623.1	$ 695.2	$ 547.6	$ 366.8	$ 403.4	$ 343.3	$ 303.9	$ 300.2	$ 297.1	$ 215.9	$ 183.9	$ 213.7	$ 206.4	$ 172.3	$ 145.1	$ 138.6	$ 136.2	$ 132.6	$ 124.8	$ 1,695.9	$ 1,513.7	$ 403.4	$ 478.3	$ 558.9	$ 519.2	$ 86.3	$ 76.8	$ 12.3	$ 11.4	$ 19.1	$ 17.2	$ 34.2	$ 45.2	$ 16.2	$ 10.5	$ 42.5	$ 38.0	$ 37.8	$ 80.0	$ 19.3	$ 310.5	$ 220.8	$ 1,033.4	$ 871.6	$ 342.0	$ 294.4	$ 60.3	$ 40.0	$ 50.9	$ 56.0	$ 61.7	$ 45.1	$ 86.9	$ 91.0	$ 9.7	$ 8.6	$ 52.5	$ 37.2	$ 7.7	$ 0.6	$ 52.7	$ 37.2	$ 39.5	$ 38.4	$ 271.8	$ 220.8	$ 3,459.7	$ 3,090.8	$ 800.0	$ 609.0	$ 717.4	$ 725.5	$ 530.4	$ 470.6	$ 528.8	$ 457.8	$ 332.1	$ 298.2	$ 273.4	$ 207.3	$ 277.6	$ 322.4	$ 345.6	$ 251.9	$ 622.6	$ 496.2	$ 296.8	$ 330.2	$ 247.4	$ 213.6	$ 197.1	$ 195.6	$ 163.7	$ 137.3	$ 123.4	$ 89.2	$ 164.6	$ 144.5	$ 85.9	$ 99.0	$ 93.1	$ 67.1	$ 1,113.6	$ 1,072.1
Amortized Cost		$ 4,719.9	$ 4,465.3	$ 724.0	$ 620.7	$ 653.8	$ 545.1	$ 336.2	$ 401.0	$ 316.9	$ 300.5	$ 281.5	$ 300.9	$ 198.9	$ 185.8	$ 199.7	$ 204.6	$ 163.0	$ 145.5	$ 128.8	$ 128.5	$ 131.4	$ 125.4	$ 1,588.6	$ 1,504.4																																														$ 3,242.8	$ 3,085.9	$ 748.7	$ 607.2	$ 669.7	$ 718.9	$ 500.7	$ 471.3	$ 493.5	$ 456.8	$ 307.2	$ 302.1	$ 257.6	$ 206.5	$ 265.4	$ 323.1
Number of outside managers who manage Companys investment portfolio	4