Related Party Transactions - Schedule of related party transactions activity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 management_company	Mar. 31, 2012 management_company
Related Party Transaction [Line Items]
Reported net premiums written	$ 2,107,654	$ 2,041,724
Net losses and loss expenses incurred	843,084	854,065
Reported acquisition costs	220,259	224,151
Number of investment management companies	4	4
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	12,587	16,589
Net losses and loss expenses incurred	5,649	8,123
Reported acquisition costs	$ 5,330	$ 6,787