Losses and Loss Expenses (Details) - Narrative - Other information (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Loss And Loss Expenses Disclosure [Line Items]
Increase in current year net losses incurred vs prior year	$ (281,900,000)	$ 778,600,000
Increase in current year loss ratio compared to prior year	(11.10%)	10.70%
Exchange rate effects	156,206,000	(130,533,000)	(125,107,000)
Total net paid losses	3,783,242,000	3,846,202,000	3,470,509,000
Total allowance for uncollectible reinsurance relating to bothreinsurance balances receivable and unpaid losses and loss expenses recoverable	107,900,000	99,200,000
Workers' compensation
Loss And Loss Expenses Disclosure [Line Items]
Discount rate used on unpaid losses for workers compensation	5.00%
Undiscounted tabular unpaid losses and loss expenses, net of reinsurance reserve	645,200,000	612,900,000
Related unpaid losses and loss expenses, discounted	343,000,000	290,300,000
UK motor liabilities
Loss And Loss Expenses Disclosure [Line Items]
Discount rate used on unpaid losses	1.50%	2.00%
Undiscounted tabular unpaid losses and loss expenses, net of reinsurance reserve	240,000,000	52,800,000
Related unpaid losses and loss expenses, discounted	$ 148,600,000	$ 29,600,000