Commitments and contingencies (Investment commitments) (Details) - USD ($) $ in Thousands					12 Months Ended
	Dec. 30, 2015	Dec. 29, 2014	Nov. 07, 2014	Oct. 02, 2014	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Aquiline Capital Partners II GP Offshore Ltd
Related Party Transaction [Line Items]
Capital commitment					$ 50,000
Remaining commitment					3,413		$ 7,500
Fixed maturities commitment
Related Party Transaction [Line Items]
Other commitment					34,888		7,539
Investment commitment
Related Party Transaction [Line Items]
Contractual obligation					263,000		153,000
Other commitment					185,548		83,712
Aquiline Financial Services Fund III LP
Related Party Transaction [Line Items]
Capital commitment			$ 100,000
Remaining commitment					86,110		100,000
AlphaCat sidecar
Related Party Transaction [Line Items]
Capital commitment		$ 28,000
Remaining commitment					0		2,400
AlphaCat Fund A
Related Party Transaction [Line Items]
Capital commitment	$ 20,000	$ 20,000
Remaining commitment						$ 10,000	8,000
AlphaCat Fund B
Related Party Transaction [Line Items]
Capital commitment	$ 25,000
Remaining commitment					9,536
Western World Insurance Group, Inc | Aquiline Capital Partners II GP Offshore Ltd
Related Party Transaction [Line Items]
Capital commitment				$ 10,000
Remaining commitment					$ 683		$ 1,499