Comprehensive income (loss) (Details 14) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	$ (38,205)	$ (48,721)
Other comprehensive income (loss) before reclassifications	(12,725)	12,452
Amounts reclassified from accumulated other comprehensive loss	1,625	(1,936)
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	7,202
Net current-period other comprehensive income (loss)	(3,898)	10,516	(1,720)
Balance	(42,103)	(38,205)	(48,721)
Net unrealized gain (loss) on derivative instruments qualifying as hedges
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(3,765)	6,018
Other comprehensive income (loss) before reclassifications	0	(5,594)
Amounts reclassified from accumulated other comprehensive loss	694	(4,189)
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	0
Net current-period other comprehensive income (loss)	694	(9,783)
Balance	(3,071)	(3,765)
Postretirement liability adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(33,807)	(54,347)
Other comprehensive income (loss) before reclassifications	(12,409)	18,539
Amounts reclassified from accumulated other comprehensive loss	796	2,001
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	7,202
Net current-period other comprehensive income (loss)	(4,411)	20,540
Balance	(38,218)	(33,807)
Foreign currency translation adjustment
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	(667)	(511)
Other comprehensive income (loss) before reclassifications	(162)	(299)
Amounts reclassified from accumulated other comprehensive loss	0	143
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	0
Net current-period other comprehensive income (loss)	(162)	(156)
Balance	(829)	(667)
Net unrealized gains on available-for-sale investments
Accumulated other comprehensive income (loss) [Roll Forward]
Balance	34	119
Other comprehensive income (loss) before reclassifications	(154)	(194)
Amounts reclassified from accumulated other comprehensive loss	135	109
Amounts reclassified from accumulated other comprehensive loss to a regulatory asset	0
Net current-period other comprehensive income (loss)	(19)	(85)
Balance	$ 15	$ 34

Reclassification from accumulated other comprehensive income to regulatory asset current period, net of tax.

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- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 14

-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 14A

-URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 31

-Article 5

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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- Definition

Amount after tax, before reclassification adjustments of other comprehensive income (loss).

+ References

No definition available.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

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-Name Accounting Standards Codification

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-Section 45

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-URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669619-108580

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

+ References

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