Related Party Transactions - Schedule of related party transactions activity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 management_company	Mar. 31, 2014	Dec. 31, 2014 management_company
Related Party Transaction [Line Items]
Reported net premiums written	$ 1,851,249	$ 1,995,851
Net losses and loss expenses incurred	769,827	831,505
Reported acquisition costs	153,696	199,414
Number of investment management companies	6		5
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	27,076	16,240
Net losses and loss expenses incurred	9,100	6,138
Reported acquisition costs	$ 9,012	$ 7,629