Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves(Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period	$ 10,296,821	$ 9,378,987	$ 10,200,960	$ 9,125,250
Unpaid losses and loss adjustment expenses recoverable	2,095,130	1,937,724	2,083,575	1,828,837
Net reserve for losses and loss adjustment expenses at beginning of period	8,201,691	7,441,263	8,117,385	7,296,413
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year	759,261	670,381	1,395,037	1,249,359
Prior years	(69,401)	(85,789)	(152,607)	(141,818)
Total net incurred losses and loss adjustment expenses	689,860	584,592	1,242,430	1,107,541
Net foreign exchange losses (gains)	75,295	(48,328)	106,574	(14,733)
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year	(80,499)	(107,957)	(115,502)	(157,036)
Prior years	(482,046)	(401,691)	(946,586)	(764,306)
Total net paid losses and loss adjustment expenses	(562,545)	(509,648)	(1,062,088)	(921,342)
Net reserve for losses and loss adjustment expenses at end of period	8,404,301	7,467,879	8,404,301	7,467,879
Unpaid losses and loss adjustment expenses recoverable	2,116,210	2,003,768	2,116,210	2,003,768
Reserve for losses and loss adjustment expenses at end of period	$ 10,520,511	$ 9,471,647	$ 10,520,511	$ 9,471,647