Debt and financing arrangements (Summary of key terms of senior notes and debentures) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Debt Instruments [Line Items]
Commitment		$ 250,000	$ 250,000
Debentures payable		$ 539,572	539,158
2006 Junior Subordinated
Debt Instruments [Line Items]
Issuance date		Jun. 15, 2006
Maturity date		Jun. 15, 2036
Interest payments due		Quarterly
Interest rate term		5 years
Debentures payable		$ 150,000	150,000
2006 Junior Subordinated | Three month LIBOR
Debt Instruments [Line Items]
Spread on variable rate (in percent)		3.55%
Flagstone 2006 Junior Subordinated
Debt Instruments [Line Items]
Issuance date		Aug. 23, 2006
Maturity date		Sep. 15, 2036
Interest payments due		Quarterly
Debentures payable		$ 136,022	135,608
2007 Junior Subordinated
Debt Instruments [Line Items]
Issuance date		Jun. 21, 2007
Commitment		$ 200,000
Maturity date		Jun. 15, 2037
Interest payments due		Quarterly
Interest rate term		5 years
Debentures payable		$ 139,800	139,800
2007 Junior Subordinated | Three month LIBOR
Debt Instruments [Line Items]
Spread on variable rate (in percent)		2.95%
Flagstone 2007 Due July 30 2037
Debt Instruments [Line Items]
Issuance date		Jun. 08, 2007
Commitment		$ 100,000
Maturity date		Jul. 30, 2037
Interest payments due		Quarterly
Debentures payable		$ 113,750	$ 113,750
Flagstone 2007 Due September 15 2037
Debt Instruments [Line Items]
Issuance date		Sep. 20, 2007
Commitment		$ 25,000
Maturity date		Sep. 15, 2037
Interest payments due		Quarterly
2010 Senior Notes Due 2040
Debt Instruments [Line Items]
Issuance date		Jan. 26, 2010
Commitment		$ 250,000
Maturity date		Jan. 26, 2040
Interest payments due		Semi-annually in arrears
At Issuance | 2006 Junior Subordinated
Debt Instruments [Line Items]
Fixed interest rate (in percent)	[1]	9.069%
At Issuance | Flagstone 2006 Junior Subordinated | Three month LIBOR
Debt Instruments [Line Items]
Spread on variable rate (in percent)	[2]	3.54%
At Issuance | 2007 Junior Subordinated
Debt Instruments [Line Items]
Fixed interest rate (in percent)	[3]	8.48%
At Issuance | Flagstone 2007 Due July 30 2037 | Three month LIBOR
Debt Instruments [Line Items]
Spread on variable rate (in percent)	[2]	3.00%
At Issuance | Flagstone 2007 Due September 15 2037 | Three month LIBOR
Debt Instruments [Line Items]
Spread on variable rate (in percent)	[2]	3.10%
At Issuance | 2010 Senior Notes Due 2040
Debt Instruments [Line Items]
Fixed interest rate (in percent)	[4]	8.875%
Outstanding | 2006 Junior Subordinated
Debt Instruments [Line Items]
Effective percentage	[5]	5.831%
Outstanding | Flagstone 2006 Junior Subordinated
Debt Instruments [Line Items]
Effective percentage	[5]	6.463%
Outstanding | 2007 Junior Subordinated
Debt Instruments [Line Items]
Effective percentage	[5]	5.18%
Outstanding | Flagstone 2007 Due July 30 2037
Debt Instruments [Line Items]
Effective percentage	[5]	5.90%
Outstanding | Flagstone 2007 Due September 15 2037
Debt Instruments [Line Items]
Effective percentage	[5]	5.983%
Outstanding | 2010 Senior Notes Due 2040
Debt Instruments [Line Items]
Fixed interest rate (in percent)	[4]	8.875%

[1] Fixed interest rate for 5 years, floating interest rate of three-month LIBOR plus 3.550% thereafter, reset quarterly.

[2] Floating interest rate of three-month LIBOR plus amount stated, reset quarterly

[3] Fixed interest rate for 5 years, floating interest rate of three-month LIBOR plus 2.950% thereafter, reset quarterly.

[4] Fixed interest rate.

[5] Fixed interest rate as a result of interest rate swap contracts entered into by the Company.