Reinsurance (Reinsurance Financial Information) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance Disclosures [Abstract]
Written premiums, Direct	$ 4,370,092,000	$ 3,788,251,000	$ 3,599,836,000
Written premiums, Assumed	707,221,000	627,826,000	653,603,000
Written premiums, Ceded	(719,945,000)	(565,151,000)	(523,344,000)
Total net premiums written	4,357,368,000	3,850,926,000	3,730,095,000
Earned premiums, Direct	4,164,277,000	3,744,150,000	3,690,493,000
Earned premiums, Assumed	669,593,000	652,485,000	617,143,000
Earned premiums, Ceded	(673,003,000)	(561,053,000)	(501,787,000)
Net premiums earned	4,160,867,000	3,835,582,000	3,805,849,000
Ceded losses incurred	458,249,000	379,153,000	252,299,000
Uncollectible reinsurance	$ 3,169,000	$ 3,098,000	$ 4,430,000