Impact of Catastrophe Costs (Details 2) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net Catastrophe Cost Components
Total net losses and LAE incurred	$ 681,126,000	$ 651,543,000	$ 1,850,178,000	$ 1,437,867,000
Net catastrophe costs	66,400,000	26,900,000	611,800,000	157,300,000
Losses and LAE incurred components
Losses and loss adjustment expenses incurred from catastrophe events: Gross	811,152,000	774,469,000	2,084,680,000	1,573,179,000
Losses and loss adjustment expenses incurred from catastrophe events: Ceded	(130,026,000)	(122,926,000)	(234,502,000)	(135,312,000)
Total net losses and LAE incurred	681,126,000	651,543,000	1,850,178,000	1,437,867,000
Reinstatement premiums:
Net catastrophe costs	66,400,000	26,900,000	611,800,000	157,300,000
Catastrophe
Net Catastrophe Cost Components
Net losses and loss and loss adjustment expenses incurred from catastrophe events occurring in: Current year	66,500,000	32,400,000	640,700,000	167,800,000
Net losses and loss and loss adjustment expenses incurred from catastrophe events occurring in: Prior year	(1,600,000)	(6,500,000)	(5,600,000)	(6,500,000)
Total net losses and LAE incurred	64,900,000	25,900,000	635,100,000	161,300,000
Net ceded (assumed) reinstatement premiums	1,500,000	1,000,000	(23,300,000)	(4,000,000)
Losses and LAE incurred components
Losses and loss adjustment expenses incurred from catastrophe events: Gross	62,000,000	83,200,000	723,400,000	218,600,000
Losses and loss adjustment expenses incurred from catastrophe events: Ceded	2,900,000	(57,300,000)	(88,300,000)	(57,300,000)
Total net losses and LAE incurred	64,900,000	25,900,000	635,100,000	161,300,000
Reinstatement premiums:
Reinstatement premiums: Gross	(300,000)	(4,300,000)	(29,200,000)	(9,300,000)
Reinstatement premiums: Ceded	1,800,000	5,300,000	5,900,000	5,300,000
Reinstatement premiums: Net	$ 1,500,000	$ 1,000,000	$ (23,300,000)	$ (4,000,000)