Reserve for Losses and Loss Expenses (Schedule of Reconciliation of Unpaid Losses and Loss Expenses) (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Reserve for Losses and Loss Expenses
Gross liability at beginning of period	$ 5,100,643	$ 4,853,359	$ 4,879,188	$ 4,761,772
Reinsurance recoverable at beginning of period	(975,523)	(920,480)	(927,588)	(919,991)
Net liability at beginning of period	4,125,120	3,932,879	3,951,600	3,841,781
Commutation of variable-rated reinsurance contracts	11,529		11,529	8,864
Current year	279,513	252,816	628,315	550,062
Prior years	(55,229)	(64,094)	(99,579)	(138,050)
Total incurred	235,813	188,722	540,265	420,876
Current year	19,579	26,704	21,279	33,410
Prior years	109,238	102,690	243,596	231,990
Total paid	128,817	129,394	264,875	265,400
Foreign exchange revaluation	5,237	(4,207)	10,363	(9,257)
Net liability at end of period	4,237,353	3,988,000	4,237,353	3,988,000
Reinsurance recoverable at end of period	1,013,951	932,435	1,013,951	932,435
Gross liability at end of period	5,251,304	4,920,435	5,251,304	4,920,435
Net Gains Incurred Related To Commutation Of Variable Rated Reinsurance Contracts	$ 865		$ 865