Variable interest entities (Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) funds subsidiaries	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) funds subsidiaries	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Variable Interest Entity [Line Items]
(Income) attributable to AlphaCat investors	$ 5,564	$ 1,438	$ 16,278	$ 1,438
Accounts payable and accrued expenses
Variable Interest Entity [Line Items]
(Income) attributable to AlphaCat investors			$ 18,690		$ 2,412
Variable interest entities, primary beneficiary
Variable Interest Entity [Line Items]
Variable funding notes, typical minimum duration			12 months
AlphaCat ILS funds
Variable Interest Entity [Line Items]
Number of AlphaCat ILS funds | funds	1		1
AlphaCat ILS funds | Variable interest entities, primary beneficiary
Variable Interest Entity [Line Items]
Risk profile percentage			7.00%
BetaCat ILS funds | Variable interest entities, primary beneficiary
Variable Interest Entity [Line Items]
Number of consolidated VIE BetaCat ILS funds | subsidiaries	1		1
Structured Notes
Variable Interest Entity [Line Items]
Structured notes interest rate percentage			8.00%