Losses and Loss Expenses - Net losses and loss expenses incurred (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Abstract]
Loss and loss expenses payments	$ 6,828,012	$ 6,505,075	$ 4,499,642
Change in unpaid losses and loss expenses	976,401	(168,263)	(514,406)
Change in unpaid losses and loss expenses recoverable	(238,086)	(440,189)	(48,536)
Paid loss recoveries	(1,493,492)	(1,130,423)	(678,307)
Net losses and loss expenses incurred	$ 6,072,835	$ 4,766,200	$ 3,258,393