Variable interest entities (Notes Payable to AlphaCat Investors) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($) funds	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) funds	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Variable Interest Entity [Line Items]
Income attributable to AlphaCat investors	$ 7,080	$ 5,564	$ 6,114	$ 4,600	$ 974	$ 1,438	$ 0	$ 0	$ 23,358	$ 2,412	$ 0
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Notes payable to AlphaCat investors, beginning of year				75,493				0	75,493	0
Issuance of notes payable to AlphaCat investors									406,037	75,770
Variable Interest Entity Redemption Of Notes Payable To AlphaCat Investors									204,038
Foreign exchange losses									710	(277)
Notes payable to AlphaCat investors, end of year	$ 278,202				75,493				$ 278,202	75,493	0
Variable Funding Notes, Typical Minimum Duration									12 months
AlphaCat ILS Funds
Variable Interest Entity [Line Items]
Number Of Funds | funds	1								1
AlphaCat ILS Funds | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Risk Profile Percentage									7.00%
Variable Funding Note [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Notes payable to AlphaCat investors, beginning of year				75,493				0	$ 75,493	0
Issuance of notes payable to AlphaCat investors									311,711	75,770
Variable Interest Entity Redemption Of Notes Payable To AlphaCat Investors									109,712
Foreign exchange losses									710	(277)
Notes payable to AlphaCat investors, end of year	$ 278,202				75,493				$ 278,202	75,493	0
Structured Note [Member]
Variable Interest Entity [Line Items]
Structured Notes Interest Rate Percentage									8.00%
Structured Note [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Notes payable to AlphaCat investors, beginning of year				$ 0				$ 0	$ 0	0
Issuance of notes payable to AlphaCat investors									94,326	0
Variable Interest Entity Redemption Of Notes Payable To AlphaCat Investors									94,326
Foreign exchange losses									0	0
Notes payable to AlphaCat investors, end of year	$ 0				$ 0				0	0	$ 0
Accounts payable and accrued expenses
Variable Interest Entity [Line Items]
Variable Interest Entity Redemption Of Notes Payable To AlphaCat Investors									1,000
Income attributable to AlphaCat investors									$ 17,068	$ 2,412