Related Party Transactions - Schedule of related party transactions activity (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 management_company	Dec. 31, 2013 management_company	Dec. 31, 2012
Related Party Transaction [Line Items]
Reported net premiums written	$ 5,944,841	$ 6,199,404	$ 6,281,451
Net losses and loss expenses incurred	3,258,393	3,731,464	3,765,482
Reported acquisition costs	738,118	909,323	913,492
Number of investment management companies	6	5
Other strategic affiliates or subsidiaries
Related Party Transaction [Line Items]
Reported net premiums written	91,261	73,085	61,146
Net losses and loss expenses incurred	38,927	29,042	37,664
Reported acquisition costs	$ 35,711	$ 25,275	$ 24,014