Colstrip Acquisition and Requests for Cost Recovery [Line Items]

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- Definition

Schedule of Regulated Operations [Line Items]

+ References

No definition available.

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- Definition

Amount of consideration transferred in asset acquisition. Includes, but is not limited to, cash, liability incurred by acquirer, and equity interest issued by acquirer.

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- Definition

Amount of indemnification asset recognized at acquisition date in asset acquisition.

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- Definition

Current portion of 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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Noncurrent portion of 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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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

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- Definition

Amount of disallowed costs for rate making purposes for recently completed plants in regulated industries.

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- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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- Definition

Amount of operating expense for routine plant maintenance, repairs and operations of regulated operation.

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Regulatory Matters (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Regulatory Matters (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
Schedule of Regulated Operations [Line Items]
Public Utilities, Property, Plant and Equipment, Amount of Disallowed Costs for Recently Completed Plant					$ 30,900
Colstrip Acquisition and Requests for Cost Recovery [Line Items]
Asset Acquisition, Consideration Transferred			$ 0
Operating, general, and administrative	$ 79,095	$ 62,336	$ 153,635	$ 119,045
Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 Acquired from Puget [Member]
Colstrip Acquisition and Requests for Cost Recovery [Line Items]
Operating, general, and administrative	$ 30,000
Colstrip Ownership in Unit 3 and 4, effective January 1, 2026 [Member]
Colstrip Acquisition and Requests for Cost Recovery [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	55.00%		55.00%
Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista
Colstrip Acquisition and Requests for Cost Recovery [Line Items]
Operating, general, and administrative	$ 18,000
Revenue Subject to Refund \| Northwestern Corporation
Schedule of Regulated Operations [Line Items]
Regulatory Liability	3,600		$ 3,600
Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista Corporation [Member]
Colstrip Acquisition and Requests for Cost Recovery [Line Items]
Asset Retirement Obligation, Current	2,800		2,800
Asset Retirement Obligations, Noncurrent	34,200		34,200
Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista Corporation [Member] \| Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsCurrent
Colstrip Acquisition and Requests for Cost Recovery [Line Items]
Asset Acquisition, Indemnification Asset, Amount	2,800		2,800
Colstrip Ownership in Unit 3 and 4, January 1, 2026 Acquired from Avista Corporation [Member] \| Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent
Colstrip Acquisition and Requests for Cost Recovery [Line Items]
Asset Acquisition, Indemnification Asset, Amount	$ 34,200		$ 34,200