Losses and Loss Expenses - Net losses and loss expenses incurred (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Abstract]
Loss and loss expenses payments	$ 8,037,317	$ 6,828,012	$ 6,505,075
Change in unpaid losses and loss expenses	3,184,307	976,401	(168,263)
Change in unpaid losses and loss expenses recoverable	(1,646,432)	(238,086)	(440,189)
Paid loss recoveries	(1,573,272)	(1,493,492)	(1,130,423)
Net losses and loss expenses incurred	$ 8,001,920	$ 6,072,835	$ 4,766,200