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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Premiums written:
Direct	$ 453,768,000	$ 431,312,000	$ 1,324,705,000	$ 1,274,061,000
Assumed	22,575,000	15,372,000	29,765,000	21,561,000
Ceded premiums written	(79,511,000)	(79,570,000)	(221,300,000)	(206,893,000)
Net	396,832,000	367,114,000	1,133,170,000	1,088,729,000
Premiums earned:
Direct	425,231,000	413,759,000	1,257,087,000	1,238,912,000
Assumed	7,626,000	9,158,000	18,866,000	20,858,000
Ceded premiums earned	(73,894,000)	(68,208,000)	(210,067,000)	(196,669,000)
Net	358,963,000	354,709,000	1,065,886,000	1,063,101,000
Losses and loss expenses incurred:
Direct	638,219,000	277,111,000	1,204,586,000	834,431,000
Assumed	5,977,000	6,612,000	13,549,000	11,535,000
Ceded losses and loss expenses incurred	(338,238,000)	(38,704,000)	(388,416,000)	(106,824,000)
Net	305,958,000	245,019,000	829,719,000	739,142,000
Increase of direct losses and loss expenses	361,100,000		370,200,000
Increase in ceded losses and loss expenses	299,500,000		281,600,000
Increase in catastrophe losses	55,500,000		60,200,000
Catastrophe losses	67,400,000		112,400,000
Hurricane Irene losses on a net basis	40,400,000
Hurricane Irene losses on a gross basis	48,100,000
Prior period storm development	$ 10,400,000

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