Investments in affiliates (Notes payable and (income) attributable to operating affiliates) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Schedule of Equity Method Investments [Line Items]
Notes payable to operating affiliates, beginning of period	$ 439,272	$ 0
Notes payable recognized on deconsolidation of AlphaCat ILS fund	178,837
Issuance of notes payable to operating affiliates	334,444	294,349
Redemption of notes payable to operating affiliates	(391,115)	0
Foreign exchange gain (loss)	(65)	(3,761)
Notes payable to operating affiliates, end of period	561,373	290,588
Variable interest entity, not primary beneficiary | AlphaCat 2013
Schedule of Equity Method Investments [Line Items]
Notes payable to operating affiliates, beginning of period	223,809	0
Notes payable recognized on deconsolidation of AlphaCat ILS fund	0
Issuance of notes payable to operating affiliates	0	162,506
Redemption of notes payable to operating affiliates	(175,349)
Foreign exchange gain (loss)	(297)	(1,067)
Notes payable to operating affiliates, end of period	48,163	161,439
Variable interest entity, not primary beneficiary | AlphaCat 2014
Schedule of Equity Method Investments [Line Items]
Notes payable to operating affiliates, beginning of period	0
Issuance of notes payable to operating affiliates	149,707
Redemption of notes payable to operating affiliates	0
Foreign exchange gain (loss)	109
Notes payable to operating affiliates, end of period	149,816
Variable interest entity, not primary beneficiary | AlphaCat ILS Funds
Schedule of Equity Method Investments [Line Items]
Notes payable to operating affiliates, beginning of period	215,463	0
Notes payable recognized on deconsolidation of AlphaCat ILS fund	178,837
Issuance of notes payable to operating affiliates	184,737	131,843
Redemption of notes payable to operating affiliates	(215,766)
Foreign exchange gain (loss)	123	(2,694)
Notes payable to operating affiliates, end of period	$ 363,394	$ 129,149