Related Party Transactions - Schedule of related party transactions activity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 management_company	Mar. 31, 2013 management_company
Related Party Transaction [Line Items]
Reported net premiums written	$ 1,995,851	$ 2,107,654
Net losses and loss expenses incurred	831,505	843,084
Reported acquisition costs	199,414	220,259
Number of investment management companies	5	4
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	16,240	12,740
Net losses and loss expenses incurred	6,138	5,649
Reported acquisition costs	$ 7,629	$ 5,353