Losses and Loss Expenses - Loss Reserve Discounting (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Loss And Loss Expenses Disclosure [Line Items]
Unpaid losses and loss expenses	$ 19,353,243,000	$ 20,481,065,000	$ 20,484,121,000	$ 20,613,901,000
Workers' compensation
Loss And Loss Expenses Disclosure [Line Items]
Discount rate used on unpaid losses for workers compensation	5.00%	5.00%
Unpaid losses and loss expenses	566,200,000	609,100,000
Undiscounted tabular unpaid losses and loss expenses, net of reinsurance reserve	515,400,000	551,600,000
Tabular unpaid losses and loss expenses	266,400,000	274,600,000
UK motor liabilities
Loss And Loss Expenses Disclosure [Line Items]
Discount rate used on unpaid losses	1.50%	1.50%
Undiscounted tabular unpaid losses and loss expenses, net of reinsurance reserve	249,800,000	262,000,000
Tabular unpaid losses and loss expenses	$ 161,000,000	$ 165,700,000