Utility Plant - Asset Retirement Obligation (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 06, 2016
number of agreements			2
Subsidiaries [Member]
Decommissioning Liability, Noncurrent	$ 4,800	$ 3,800
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement obligation at beginning of period	191,176	200,345
Costs Incurred, Asset Retirement Obligation Incurred	501	2,881
Asset Retirement Obligation, Liabilities Incurred	(4,750)	(3,841)
Asset Retirement Obligation, Revision of Estimate	(10,512)	(13,748)
Asset Retirement Obligation, Accretion Expense	5,788	5,539
Asset retirement obligation at end of period	182,203	191,176
Colstrip Units 1 and 2 [Member] \| Subsidiaries [Member]
Decommissioning Liability, Noncurrent	5,500	11,000
Colstrip Units 3 and 4 [Member] \| Subsidiaries [Member]
Decommissioning Liability, Noncurrent	12,700	1,800
Tacoma LNG [Member]
Decommissioning Liability, Noncurrent		2,700
Tacoma LNG [Member] \| Puget LNG [Member]
Decommissioning Liability, Noncurrent	$ 1,700	$ 2,200

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- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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-Paragraph 1
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Amount of asset retirement obligations incurred during the period.

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Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
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-Paragraph 8
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of asset retirement obligations (ARO) costs incurred (whether capitalized or charged to expense) during the period.

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- Definition

Amount of a decommissioning liability associated with a legal obligation that a firm incurs when it constructs or operates an asset that must eventually be withdrawn from active service. This may include operating assets such as nuclear power plants or shipping vessels.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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