Debt and financing arrangements (Components of finance expenses) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of financial expenses
Finance expenses		$ 15,871	$ 14,523	$ 14,209	$ 13,943	$ 14,630	$ 14,521	$ 14,166	$ 15,203	$ 58,546	$ 58,520	$ 74,742
2006 Junior Subordinated Deferrable Debentures [Member]
Components of financial expenses
Finance expenses										8,868	8,893	8,868
2007 Junior Subordinated Deferrable Debentures [Member]
Components of financial expenses
Finance expenses										7,342	7,362	7,341
Flagstone 2006 Junior Subordinated Deferrable Interest Notes [Member]
Components of financial expenses
Finance expenses										9,012	9,028	8,989
Flagstone 2007 Junior Subordinated Deferrable Interest Notes [Member]
Components of financial expenses
Finance expenses										7,013	7,100	7,123
2010 Senior Notes due 2040
Components of financial expenses
Finance expenses										22,389	22,388	22,388
Credit Facilities [Member]
Components of financial expenses
Finance expenses										2,014	2,060	6,006
Bank charges, Talbot FAL facility and other charges
Components of financial expenses
Finance expenses	[1]									1,766	504	4,592
AlphaCat Fees [Member]
Components of financial expenses
Finance expenses	[2]									$ 142	$ 1,185	$ 9,435

[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 in relation to fund raising for the AlphaCat sidecars, the AlphaCat ILS funds and AlphaCat direct.