Variable interest entities (Assets and Liabilities of Consolidated VIEs) (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) subsidiaries	Dec. 31, 2014 USD ($)
Variable Interest Entity [Line Items]
Variable Funding Notes, Typical Minimum Duration		12 months
AlphaCat Sidecars | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets		$ 206,581	$ 389,910
Total liabilities		14,804	37,542
AlphaCat ILS Funds - Lower Risk (a) | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets	[1]	1,268,070	856,980
Total liabilities	[1]	143,371	161,406
AlphaCat ILS Funds - Higher Risk (a) | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets	[1]	522,867	173,646
Total liabilities	[1]	300,122	18,371
AlphaCat Re & Master Fund | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets		1,615,779	946,723
Total liabilities		$ 1,615,609	946,553
BetaCat ILS Funds | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Number of Consolidated Funds | subsidiaries		1
Total assets		$ 64,221	16,189
Total liabilities		$ 2,472	$ 278

[1] Lower risk AlphaCat ILS funds have a maximum permitted portfolio expected loss of less than 7%, whereas higher risk AlphaCat ILS funds have a maximum permitted portfolio expected loss of greater than 7%. Expected loss represents the average annual loss over the set of simulation scenarios divided by the total limit.