Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) management_company	Dec. 31, 2015 USD ($) management_company	Dec. 31, 2014 USD ($)
Related Party Transaction [Line Items]
Reported net premiums written	$ 10,242,683	$ 7,951,063	$ 5,944,841
Net losses and loss expenses incurred	6,072,835	4,766,200	3,258,393
Reported acquisition costs	$ 1,620,671	$ 1,317,448	738,118
Number of investment management companies | management_company	4	6
Other strategic affiliates or subsidiaries
Related Party Transaction [Line Items]
Reported net premiums written	$ 94,358	$ 42,042	91,261
Net losses and loss expenses incurred	44,728	13,246	38,927
Reported acquisition costs	$ 39,131	$ 14,682	$ 35,711