Losses and Loss Expenses - Loss Reserve Discounting (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss And Loss Expenses Disclosure [Line Items]
Unpaid losses and loss expenses	$ 25,939,571	$ 25,439,744	$ 19,353,243	$ 20,481,065
Liabilities for unpaid losses and loss expenses, net of reinsurance	20,052,443
Bodily Injury Liability Claims, Discount, Amount	125,900	130,200
Workers' Compensation Discount, Amount	$ 287,700	302,100
Workers' compensation
Loss And Loss Expenses Disclosure [Line Items]
Discount rate used on unpaid losses for workers compensation	3.75%
Liabilities for unpaid losses and loss expenses, net of reinsurance	$ 727,400	747,400
Tabular unpaid losses and loss expenses	439,700	445,300
Interest accretion related to the unwind of discounted reserves	$ 18,600	$ 7,800
UK motor liabilities
Loss And Loss Expenses Disclosure [Line Items]
Discount rate used on unpaid losses	2.00%	2.00%
Liabilities for unpaid losses and loss expenses, net of reinsurance	$ 281,600	$ 298,100
Tabular unpaid losses and loss expenses	155,700	167,900
Periodic Payment Orders Related To Bodily Injury Claims [Member]
Loss And Loss Expenses Disclosure [Line Items]
Interest accretion related to the unwind of discounted reserves	$ 2,800	$ 2,600