Environmental matters (Details 2) - USD ($)		6 Months Ended
Oct. 12, 2015	Jun. 30, 2016
Portland Harbor Site [Member]
Site Contingency [Line Items]
Environmental matters investigative costs		$ 70,000,000
Coos County Oregon [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate	$ 160,000
Eugene, OR Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, low estimate, loss exposure		500,000
Site contingency, high estimate, loss exposure		$ 11,000,000
Estimated proportional share of cleanup liability		50.00%
Accrual for environmental loss contingencies		$ 1,700,000
Bremerton, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, low estimate, loss exposure		340,000
Site contingency, high estimate, loss exposure		6,400,000
Accrual for environmental loss contingencies		12,800,000
Bellingham, WA Manufactured Gas Plant Site [Member]
Site Contingency [Line Items]
Site contingency, loss exposure not accrued, best estimate		$ 8,000,000

Estimated investigative costs.

No definition available.

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- Definition

The estimated proportional share of the clean-up costs.

+ References

No definition available.

+ Details

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- Definition

High-end of the range estimate of the loss exposure for reasonably possible environmental contingencies at an individual site.

+ References

No definition available.

+ Details

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- Definition

Low-end of the range estimate of the loss exposure for reasonable possible environmental contingencies at an individual site.

+ References

No definition available.

+ Details

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- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Best estimate of the loss exposure for reasonably possible environmental contingencies at an individual site for which no accrual has been recorded.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 10
-Subparagraph (c),(d)(2)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

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