Commitments and contingencies (Investment commitments) (Details) - USD ($) $ in Thousands						12 Months Ended
	Dec. 30, 2016	Dec. 30, 2015	Dec. 29, 2014	Nov. 07, 2014	Oct. 02, 2014	Dec. 31, 2016	Dec. 31, 2015
Aquiline Capital Partners II GP Offshore Ltd
Related Party Transaction [Line Items]
Capital commitment						$ 50,000
Remaining commitment						1,700	$ 3,413
Fixed maturities commitment
Related Party Transaction [Line Items]
Other commitment						28,499	34,888
Investment commitment
Related Party Transaction [Line Items]
Contractual obligation						308,000	263,000
Remaining commitment						156,134	185,548
Aquiline Financial Services Fund III LP
Related Party Transaction [Line Items]
Capital commitment				$ 100,000
Remaining commitment						62,031	86,110
AlphaCat Fund A
Related Party Transaction [Line Items]
Capital commitment			$ 20,000
Remaining commitment						0	10,000
Fund B - initial investment
Related Party Transaction [Line Items]
Capital commitment		$ 25,000
Remaining commitment						0	9,536
AlphaCat Fund B
Related Party Transaction [Line Items]
Capital commitment	$ 25,000
Remaining commitment						10,000
Western World Insurance Group, Inc [Member] | Aquiline Capital Partners II GP Offshore Ltd
Related Party Transaction [Line Items]
Capital commitment					$ 10,000	10,000
Remaining commitment						$ 340	$ 683