Reinsurance (Reinsurance Financial Information) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Reinsurance Disclosures [Abstract]
Written premiums, direct	$ 1,267,882,000	$ 1,114,494,000	$ 3,718,477,000	$ 3,286,220,000
Written premiums, assumed	226,842,000	191,806,000	608,693,000	535,214,000
Written premiums, ceded	(218,837,000)	(180,161,000)	(656,766,000)	(554,577,000)
Total net premiums written	1,275,887,000	1,126,139,000	3,670,404,000	3,266,857,000
Earned premiums, direct	1,208,257,000	1,070,279,000	3,475,375,000	3,063,410,000
Earned premiums, assumed	193,024,000	165,208,000	559,304,000	489,020,000
Earned premiums, ceded	(214,748,000)	(179,664,000)	(601,138,000)	(496,866,000)
Net premiums earned	1,186,533,000	1,055,823,000	3,433,541,000	3,055,564,000
Ceded losses incurred	114,273,000	134,027,000	308,692,000	329,139,000
Uncollectible reinsurance	$ 2,000,000		$ 2,000,000