Reinsurance (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance Recoverable On Paid And Unpaid Losses [Abstract]
Allowance For Uncollectible Reinsurance Recoverable	$ 12,500,000				$ 6,900,000				$ 12,500,000	$ 6,900,000
Premiums written
Assumed									4,633,054,000	4,885,266,000	4,000,888,000
Ceded									146,725,000	180,150,000	52,184,000
Net	879,900,000	1,079,600,000	1,056,500,000	1,470,400,000	820,600,000	987,600,000	1,112,700,000	1,784,200,000	4,486,329,000	4,705,116,000	3,948,704,000
Premiums Earned Net Consolidated [Abstract]
Assumed									4,789,293,000	4,956,897,000	4,202,379,000
Ceded									141,539,000	180,426,000	82,554,000
Net	1,181,400,000	1,294,300,000	1,107,500,000	1,064,600,000	1,204,600,000	1,313,400,000	1,104,600,000	1,153,800,000	4,647,754,000	4,776,471,000	4,119,825,000
Loss and Loss Expenses and Life Policy Benefits
Assumed									4,456,094,000	3,399,157,000	2,313,951,000
Ceded									83,524,000	115,539,000	18,655,000
Losses and loss expenses and life policy benefits	$ 1,069,200,000	$ 881,700,000	$ 814,500,000	$ 1,607,200,000	$ 817,800,000	$ 748,900,000	$ 704,600,000	$ 1,012,400,000	$ 4,372,570,000	$ 3,283,618,000	$ 2,295,296,000