Debt and financing arrangements (Summary of outstanding debentures and senior notes payable) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instruments [Line Items]
Debentures payable	$ 538,751	$ 539,158
Face amount	250,000	250,000
Less: Unamortized debt issuance costs	(4,336)	(4,436)
Total senior notes payable	245,664	245,564
Debentures payable	784,415	784,722
2006 Junior Subordinated
Debt Instruments [Line Items]
Debentures payable	150,000	150,000
2007 Junior Subordinated
Debt Instruments [Line Items]
Debentures payable	139,800	139,800
Face amount	200,000
Flagstone 2006 Junior Subordinated
Debt Instruments [Line Items]
Debentures payable	135,201	135,608
Flagstone 2007 Due July 30 2037
Debt Instruments [Line Items]
Debentures payable	113,750	$ 113,750
Face amount	$ 100,000