Debt and financing arrangements (Components of finance expenses) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Debt Instruments [Line Items]
Finance expenses		$ 14,521	$ 18,512	$ 43,890	$ 58,161
Credit facilities
Debt Instruments [Line Items]
Finance expenses		463	1,293	1,359	4,193
Bank charges, Talbot FAL facility and other charges
Debt Instruments [Line Items]
Finance expenses	[1]	276	1,149	489	3,608
AlphaCat fees
Debt Instruments [Line Items]
Finance expenses	[2]	144	2,309	1,105	9,374
2006 Junior Subordinated
Debt Instruments [Line Items]
Finance expenses		2,135	2,235	6,557	6,633
2007 Junior Subordinated
Debt Instruments [Line Items]
Finance expenses		1,851	1,848	5,512	5,492
Flagstone 2006 Junior Subordinated
Debt Instruments [Line Items]
Finance expenses		2,271	2,274	6,760	6,735
Flagstone 2007 Junior Subordinated
Debt Instruments [Line Items]
Finance expenses		1,784	1,807	5,317	5,335
2010 Senior Notes Due 2040
Debt Instruments [Line Items]
Finance expenses		$ 5,597	$ 5,597	$ 16,791	$ 16,791

[1] On November 30, 2015, the Company terminated its Talbot FAL Facility provided and arranged by Lloyds Bank plc and ING Bank N.V., London Branch.

[2] Includes finance expenses incurred by AlphaCat Managers Ltd. in relation to fund raising for the AlphaCat sidecars, the AlphaCat ILS funds and AlphaCat direct.