Losses and Loss Expenses (Details) - Loss Reserve Rollforward and Components of Net Losses and Loss Expenses (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and loss expenses at beginning of year	$ 20,613,901	$ 20,531,607	$ 20,823,524
Unpaid losses and loss expenses recoverable at beginning of year	3,629,928	3,649,290	3,557,391
Net unpaid losses and loss expenses at beginning of year	16,983,973	16,882,317	17,266,133
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year	4,081,377	4,363,258	3,584,662
Prior years	(315,895)	(284,867)	(372,862)
Total net incurred losses and loss expenses	3,765,482	4,078,391	3,211,800
Exchange rate effects	156,206	(130,533)	(125,107)
Less net losses and loss expenses paid in respect of losses occurring in:
Current year	416,844	589,870	442,262
Prior years	3,366,398	3,256,332	3,028,247
Total net paid losses	3,783,242	3,846,202	3,470,509
Net unpaid losses and loss expenses at end of year	17,122,419	16,983,973	16,882,317
Unpaid losses and loss expenses recoverable at end of year	3,361,702	3,629,928	3,649,290
Unpaid losses and loss expenses at end of year	20,484,121	20,613,901	20,531,607
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	3,765,482	4,078,391	3,211,800
Loss and loss expenses payments
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	4,673,997	4,911,737	4,309,523
Change in unpaid losses and loss expenses
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	(297,660)	260,631	(141,589)
Change in unpaid losses and loss expenses recoverable
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	279,900	(28,442)	(117,120)
Paid loss recoveries
Components of Net Losses And Loss Expenses Incurred [Abstract]
Net losses and loss expenses incurred	$ (890,755)	$ (1,065,535)	$ (839,014)