Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Premiums written:
Direct	$ 520,943,000	$ 453,768,000	$ 1,504,429,000	$ 1,324,705,000
Assumed	17,976,000	22,575,000	44,712,000	29,765,000
Ceded premiums written	(88,401,000)	(79,511,000)	(252,888,000)	(221,300,000)
Net	450,518,000	396,832,000	1,296,253,000	1,133,170,000
Premiums earned:
Direct	474,055,000	425,231,000	1,389,373,000	1,257,087,000
Assumed	18,595,000	7,626,000	49,683,000	18,866,000
Ceded premiums earned	(86,425,000)	(73,894,000)	(261,790,000)	(210,067,000)
Net	406,225,000	358,963,000	1,177,266,000	1,065,886,000
Losses and loss expenses incurred:
Direct	327,883,000	638,219,000	881,537,000	1,204,586,000
Assumed	13,970,000	5,977,000	35,039,000	13,549,000
Ceded losses and loss expenses incurred	(69,602,000)	(338,238,000)	(103,516,000)	(388,416,000)
Net	272,251,000	305,958,000	813,060,000	829,719,000
Number of Insurance Subsidiaries	10		10
Decrease In Property And Casualty Catastrophe Losses	57,900,000		65,700,000
Incurred Property And Casualty Catastrophe Losses	$ 9,600,000		$ 46,700,000