Reinsurance and Other Monoline Exposures (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance and Other Monoline Exposures
Reduction to net income if unearned assumed premiums are recaptured	$ 13,900,000								$ 13,900,000
Reduction to net income if additional unearned assumed premiums are recaptured	16,800,000								16,800,000
Commutation gains (losses)									32,200,000	49,800,000	(1,800,000)
Commutations of Ceded Reinsurance Contracts
Increase (decrease) in net unearned premium reserve									2,300,000	104,400,000	65,100,000
Increase (decrease) in net par									265,000,000	15,470,000,000	2,936,000,000
Cancellation of an Assured Reinsurance Contracts
Increase (decrease) in net unearned premium reserve									(22,400,000)	(84,500,000)	(31,400,000)
Increase (decrease) in net par									(1,045,000,000)	(3,097,000,000)	(894,000,000)
Premiums Written:
Direct									190,300,000	343,300,000	485,800,000
Assumed									(63,500,000)	(120,900,000)	70,600,000
Ceded									4,300,000	100,500,000	37,800,000
Net									131,100,000	322,900,000	594,200,000
Premiums Earned:
Direct									997,100,000	1,242,500,000	870,500,000
Assumed									46,300,000	72,900,000	136,400,000
Ceded									(123,300,000)	(128,700,000)	(76,500,000)
Net	225,000,000	211,100,000	230,000,000	254,000,000	286,300,000	288,700,000	297,000,000	314,700,000	920,055,000	1,186,705,000	930,429,000
Loss and LAE:
Direct									577,800,000	398,500,000	276,200,000
Assumed									4,200,000	74,200,000	135,600,000
Ceded									(120,100,000)	(60,500,000)	(18,000,000)
Net	$ 148,600,000	$ 214,900,000	$ 123,900,000	$ (25,500,000)	$ 104,800,000	$ 110,800,000	$ 85,700,000	$ 110,900,000	$ 461,890,000	$ 412,161,000	$ 393,800,000