Losses and Loss Expenses - Loss Reserve Rollforward and Components of Net Losses and Loss Expenses (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	$ 3,258,393	$ 3,731,464	$ 3,765,482
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and loss expenses at beginning of year	20,481,065	20,484,121	20,613,901
Unpaid losses and loss expenses recoverable at beginning of year	3,414,735	3,361,703	3,629,940
Net unpaid losses and loss expenses at beginning of year	17,066,330	17,122,418	16,983,961
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year	3,513,469	4,021,353	4,081,376
Prior year development	(255,076)	(289,889)	(315,894)
Total net incurred losses and loss expenses	3,258,393	3,731,464	3,765,482
Exchange rate effects	(561,673)	40,587	156,217
Less net losses and loss expenses paid in respect of losses occurring in:
Current year	381,008	425,254	416,844
Prior years	3,440,327	3,402,885	3,366,398
Total net paid losses	3,821,335	3,828,139	3,783,242
Net unpaid losses and loss expenses at end of year	15,941,715	17,066,330	17,122,418
Unpaid losses and loss expenses recoverable at end of year	3,411,528	3,414,735	3,361,703
Unpaid losses and loss expenses at end of year	19,353,243	20,481,065	20,484,121
Loss and loss expenses payments
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	4,499,642	4,496,802	4,673,998
Change in unpaid losses and loss expenses
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	(514,406)	(71,901)	(297,660)
Change in unpaid losses and loss expenses recoverable
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	(48,536)	(24,774)	279,900
Paid loss recoveries
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	$ (678,307)	$ (668,663)	$ (890,756)