Utility Plant - Schedule of Asset Retirement Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Tacoma LNG [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Decommissioning Liability, Noncurrent	$ 3,000	$ 2,700
Subsidiaries [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement obligation at beginning of the period	182,203	191,176
New asset retirement obligation recognized in the period	0	501
Relief of liability	(12,449)	(4,750)
Revisions in estimated cash flows	5,922	(10,512)
Accretion expense	5,677	5,788
Asset Retirement Obligation, Ending Balance	181,353	182,203
Decommissioning Liability, Noncurrent	12,400	4,800
Subsidiaries [Member] \| Colstrip Units 1 and 2 [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Decommissioning Liability, Noncurrent	4,200	11,000
Regulatory Assets, Unrecovered plant balance	126,500
Puget LNG [Member] \| Tacoma LNG [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Decommissioning Liability, Noncurrent	$ 4,300	$ 1,700

Regulatory Assets, Unrecovered plant balance

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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
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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.

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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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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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