Variable interest entities (Notes payable to AlphaCat investors) (Details) - Variable interest entities, primary beneficiary - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Notes payable to AlphaCat investors, beginning of period	$ 75,493	$ 0
Issuance of notes payable to AlphaCat investors	406,239	75,770
Redemption of notes payable to AlphaCat investors	(109,712)
Foreign exchange losses	710	(277)
Notes payable to AlphaCat investors, end of period	372,730	75,493
Variable Funding Notes
Variable Interest Entity [Line Items]
Notes payable to AlphaCat investors, beginning of period	75,493	0
Issuance of notes payable to AlphaCat investors	311,913	75,770
Redemption of notes payable to AlphaCat investors	(109,712)
Foreign exchange losses	710	(277)
Notes payable to AlphaCat investors, end of period	278,404	75,493
Structured Notes
Variable Interest Entity [Line Items]
Notes payable to AlphaCat investors, beginning of period	0	0
Issuance of notes payable to AlphaCat investors	94,326	0
Redemption of notes payable to AlphaCat investors	0
Foreign exchange losses	0	0
Notes payable to AlphaCat investors, end of period	$ 94,326	$ 0