Debt and financing arrangements (Summary of key terms of notes and debentures) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Amount		$ 250,000	$ 250,000
2006 Junior Subordinated Deferrable Debentures [Member]
Debt Instrument [Line Items]
Issuance date		Jun. 15, 2006
Amount		$ 150,000
Maturity date		Jun. 15, 2036
Interest payments due		Quarterly
Interest rate term		5 years
2006 Junior Subordinated Deferrable Debentures [Member] | LIBOR
Debt Instrument [Line Items]
Spread on LIBOR		3.55%
Flagstone 2006 Junior Subordinated Deferrable Interest Notes [Member]
Debt Instrument [Line Items]
Issuance date		Aug. 23, 2006
Amount		$ 133,676
Maturity date		Sep. 15, 2036
Interest payments due		Quarterly
2007 Junior Subordinated Deferrable Debentures [Member]
Debt Instrument [Line Items]
Issuance date		Jun. 21, 2007
Amount		$ 200,000
Maturity date		Jun. 15, 2037
Interest payments due		Quarterly
Interest rate term		5 years
2007 Junior Subordinated Deferrable Debentures [Member] | LIBOR
Debt Instrument [Line Items]
Spread on LIBOR		2.95%
Flagstone 2007 Due July 30 2037 [Member]
Debt Instrument [Line Items]
Issuance date		Jun. 08, 2007
Amount		$ 88,750
Maturity date		Jul. 30, 2037
Interest payments due		Quarterly
Flagstone 2007 Debt Due September 15 2037 [Member]
Debt Instrument [Line Items]
Issuance date		Sep. 20, 2007
Amount		$ 25,000
Maturity date		Sep. 15, 2037
Interest payments due		Quarterly
2010 Senior Notes due 2040 [Member]
Debt Instrument [Line Items]
Issuance date		Jan. 26, 2010
Amount		$ 250,000
Maturity date		Jan. 26, 2040
Interest payments due		Semi-annually in arrears
Debt Information At Issuance [Member] | 2006 Junior Subordinated Deferrable Debentures [Member]
Debt Instrument [Line Items]
Fixed interest rate	[1]	9.069%
Debt Information At Issuance [Member] | Flagstone 2006 Junior Subordinated Deferrable Interest Notes [Member] | LIBOR
Debt Instrument [Line Items]
Spread on LIBOR	[2]	3.54%
Debt Information At Issuance [Member] | 2007 Junior Subordinated Deferrable Debentures [Member]
Debt Instrument [Line Items]
Fixed interest rate	[3]	8.48%
Debt Information At Issuance [Member] | Flagstone 2007 Due July 30 2037 [Member] | LIBOR
Debt Instrument [Line Items]
Spread on LIBOR	[2]	3.00%
Debt Information At Issuance [Member] | Flagstone 2007 Debt Due September 15 2037 [Member] | LIBOR
Debt Instrument [Line Items]
Spread on LIBOR	[2]	3.10%
Debt Information At Issuance [Member] | 2010 Senior Notes due 2040 [Member]
Debt Instrument [Line Items]
Fixed interest rate	[4]	8.875%
Debt Outstanding [Member] | 2006 Junior Subordinated Deferrable Debentures [Member]
Debt Instrument [Line Items]
Effective interest rate	[5]	5.831%
Debt Outstanding [Member] | Flagstone 2006 Junior Subordinated Deferrable Interest Notes [Member]
Debt Instrument [Line Items]
Effective interest rate	[5]	6.463%
Debt Outstanding [Member] | 2007 Junior Subordinated Deferrable Debentures [Member]
Debt Instrument [Line Items]
Effective interest rate	[5]	5.18%
Debt Outstanding [Member] | Flagstone 2007 Due July 30 2037 [Member]
Debt Instrument [Line Items]
Effective interest rate	[5]	5.90%
Debt Outstanding [Member] | Flagstone 2007 Debt Due September 15 2037 [Member]
Debt Instrument [Line Items]
Effective interest rate	[5]	5.983%
Debt Outstanding [Member] | 2010 Senior Notes due 2040 [Member]
Debt Instrument [Line Items]
Fixed interest rate	[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.