Investments (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 manager	Mar. 31, 2012 Municipal Bonds	Dec. 31, 2011 Municipal Bonds	Mar. 31, 2012 Municipal Bonds Texas	Dec. 31, 2011 Municipal Bonds Texas	Mar. 31, 2012 Municipal Bonds New York	Dec. 31, 2011 Municipal Bonds New York	Mar. 31, 2012 Municipal Bonds California	Dec. 31, 2011 Municipal Bonds California	Mar. 31, 2012 Municipal Bonds Florida	Dec. 31, 2011 Municipal Bonds Florida	Mar. 31, 2012 Municipal Bonds Illinois	Dec. 31, 2011 Municipal Bonds Illinois	Mar. 31, 2012 Municipal Bonds Washington	Dec. 31, 2011 Municipal Bonds Washington	Mar. 31, 2012 Municipal Bonds Massachusetts	Dec. 31, 2011 Municipal Bonds Massachusetts	Mar. 31, 2012 Municipal Bonds Arizona	Dec. 31, 2011 Municipal Bonds Arizona	Dec. 31, 2011 Municipal Bonds Ohio	Mar. 31, 2012 Municipal Bonds Michigan	Dec. 31, 2011 Municipal Bonds Michigan	Mar. 31, 2012 Municipal Bonds All other states	Dec. 31, 2011 Municipal Bonds All other states	Mar. 31, 2012 Municipal Bonds Georgia	Mar. 31, 2012 Pre-refunded Bond	Dec. 31, 2011 Pre-refunded Bond	Mar. 31, 2012 State General Obligation	Dec. 31, 2011 State General Obligation	Mar. 31, 2012 State General Obligation Texas	Dec. 31, 2011 State General Obligation Texas	Mar. 31, 2012 State General Obligation New York	Dec. 31, 2011 State General Obligation New York	Mar. 31, 2012 State General Obligation California	Dec. 31, 2011 State General Obligation California	Mar. 31, 2012 State General Obligation Florida	Dec. 31, 2011 State General Obligation Florida	Mar. 31, 2012 State General Obligation Illinois	Dec. 31, 2011 State General Obligation Illinois	Mar. 31, 2012 State General Obligation Washington	Dec. 31, 2011 State General Obligation Washington	Mar. 31, 2012 State General Obligation Massachusetts	Dec. 31, 2011 State General Obligation Massachusetts	Mar. 31, 2012 State General Obligation All other states	Dec. 31, 2011 State General Obligation All other states	Mar. 31, 2012 State General Obligation Georgia	Mar. 31, 2012 Local General Obligation	Dec. 31, 2011 Local General Obligation	Mar. 31, 2012 Local General Obligation Texas	Dec. 31, 2011 Local General Obligation Texas	Mar. 31, 2012 Local General Obligation New York	Dec. 31, 2011 Local General Obligation New York	Mar. 31, 2012 Local General Obligation California	Dec. 31, 2011 Local General Obligation California	Mar. 31, 2012 Local General Obligation Florida	Dec. 31, 2011 Local General Obligation Florida	Mar. 31, 2012 Local General Obligation Illinois	Dec. 31, 2011 Local General Obligation Illinois	Mar. 31, 2012 Local General Obligation Washington	Dec. 31, 2011 Local General Obligation Washington	Mar. 31, 2012 Local General Obligation Massachusetts	Dec. 31, 2011 Local General Obligation Massachusetts	Mar. 31, 2012 Local General Obligation Arizona	Dec. 31, 2011 Local General Obligation Arizona	Dec. 31, 2011 Local General Obligation Ohio	Mar. 31, 2012 Local General Obligation Michigan	Dec. 31, 2011 Local General Obligation Michigan	Mar. 31, 2012 Local General Obligation All other states	Dec. 31, 2011 Local General Obligation All other states	Mar. 31, 2012 Local General Obligation Georgia	Mar. 31, 2012 Revenue bonds	Dec. 31, 2011 Revenue bonds	Mar. 31, 2012 Revenue bonds Tax Backed	Dec. 31, 2011 Revenue bonds Tax Backed	Mar. 31, 2012 Revenue bonds Transportation	Dec. 31, 2011 Revenue bonds Transportation	Mar. 31, 2012 Revenue bonds Municipal utilities	Dec. 31, 2011 Revenue bonds Municipal utilities	Mar. 31, 2012 Revenue bonds Water and sewer	Dec. 31, 2011 Revenue bonds Water and sewer	Mar. 31, 2012 Revenue bonds Higher Education	Dec. 31, 2011 Revenue bonds Higher Education	Mar. 31, 2012 Revenue bonds Healthcare	Dec. 31, 2011 Revenue bonds Healthcare	Mar. 31, 2012 Revenue bonds All other revenue sources	Dec. 31, 2011 Revenue bonds All other revenue sources	Mar. 31, 2012 Revenue bonds Texas	Dec. 31, 2011 Revenue bonds Texas	Mar. 31, 2012 Revenue bonds New York	Dec. 31, 2011 Revenue bonds New York	Mar. 31, 2012 Revenue bonds California	Dec. 31, 2011 Revenue bonds California	Mar. 31, 2012 Revenue bonds Florida	Dec. 31, 2011 Revenue bonds Florida	Mar. 31, 2012 Revenue bonds Illinois	Dec. 31, 2011 Revenue bonds Illinois	Mar. 31, 2012 Revenue bonds Washington	Dec. 31, 2011 Revenue bonds Washington	Mar. 31, 2012 Revenue bonds Massachusetts	Dec. 31, 2011 Revenue bonds Massachusetts	Mar. 31, 2012 Revenue bonds Arizona	Dec. 31, 2011 Revenue bonds Arizona	Dec. 31, 2011 Revenue bonds Ohio	Mar. 31, 2012 Revenue bonds Michigan	Dec. 31, 2011 Revenue bonds Michigan	Mar. 31, 2012 Revenue bonds All other states	Dec. 31, 2011 Revenue bonds All other states	Mar. 31, 2012 Revenue bonds Georgia
Investments
Fair Value		$ 5,096.1	$ 5,052.0	$ 782.2	$ 773.9	$ 716.4	$ 695.2	$ 395.6	$ 366.8	$ 338.7	$ 343.3	$ 291.8	$ 300.2	$ 232.9	$ 213.7	$ 220.3	$ 215.9	$ 178.6	$ 172.3	$ 138.6	$ 136.5	$ 136.2	$ 1,674.3	$ 1,695.9	$ 128.8	$ 430.0	$ 403.4	$ 552.4	$ 558.9	$ 86.6	$ 86.3	$ 12.5	$ 12.3	$ 18.8	$ 19.1	$ 34.1	$ 34.2	$ 14.1	$ 16.2	$ 38.1	$ 37.8	$ 42.4	$ 42.5	$ 293.3	$ 310.5	$ 12.5	$ 981.4	$ 1,033.4	$ 330.4	$ 342.0	$ 59.8	$ 60.3	$ 69.3	$ 50.9	$ 61.5	$ 61.7	$ 83.4	$ 86.9	$ 52.3	$ 52.5	$ 9.6	$ 9.7	$ 7.6	$ 7.7	$ 52.7	$ 36.9	$ 37.2	$ 241.3	$ 271.8	$ 29.3	$ 3,562.3	$ 3,459.7	$ 804.8	$ 800.0	$ 748.9	$ 717.4	$ 562.1	$ 528.8	$ 546.1	$ 530.4	$ 401.4	$ 332.1	$ 275.9	$ 273.4	$ 223.1	$ 277.6	$ 365.2	$ 345.6	$ 644.1	$ 622.6	$ 307.5	$ 296.8	$ 243.1	$ 247.4	$ 194.3	$ 197.1	$ 142.5	$ 123.4	$ 168.3	$ 163.7	$ 171.0	$ 164.6	$ 85.9	$ 99.6	$ 99.0	$ 1,139.7	$ 1,113.6	$ 87.0
Amortized Cost		$ 4,746.3	$ 4,719.9	$ 726.7	$ 724.0	$ 672.4	$ 653.8	$ 360.7	$ 336.2	$ 311.0	$ 316.9	$ 271.6	$ 281.5	$ 218.3	$ 199.7	$ 202.0	$ 198.9	$ 168.0	$ 163.0	$ 128.8	$ 128.1	$ 128.5	$ 1,564.9	$ 1,588.6	$ 122.6																																														$ 3,320.8	$ 3,242.8	$ 749.9	$ 748.7	$ 693.9	$ 669.7	$ 522.4	$ 493.5	$ 511.9	$ 500.7	$ 374.4	$ 307.2	$ 257.0	$ 257.6	$ 211.3	$ 265.4
Number of outside managers who manage Company's investment portfolio	4