Debt and financing arrangements (Summary of outstanding debentures and senior notes payable) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debentures payable	$ 539,158	$ 537,226
Debt instrument, commitment	250,000	250,000
Unamortized debt issuance costs	(4,436)	(4,638)
Senior notes payable	245,564	245,362
Debentures and senior notes payable	784,722	782,588
Junior Subordinated Deferrable Debentures Two Thousand Six [Member]
Debt Instrument [Line Items]
Debentures payable	150,000	150,000
Debt instrument, commitment	150,000
Junior Subordinated Deferrable Debentures Two Thousand Seven [Member]
Debt Instrument [Line Items]
Debentures payable	139,800	139,800
Debt instrument, commitment	200,000
Flagstone 2007 Due July 30 2037 [Member]
Debt Instrument [Line Items]
Debentures payable	113,750	113,750
Debt instrument, commitment	100,000
Flagstone 2006 Junior Subordinated Deferrable Interest Notes [Member]
Debt Instrument [Line Items]
Debentures payable	135,608	$ 133,676
Debt instrument, commitment	$ 135,608

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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-Subparagraph (SX 210.7-03(a)(16))
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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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