Long-term private shelf agreement amended issuance capacity.

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Ratio of funded debt to total capitalization as specified in debt convenants.

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Ratio of total debt to total capitalization as specified in debt convenants.

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Ratio of total debt to total capitalization for the current period ended.

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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.

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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 210
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-Paragraph 1
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
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-Name Accounting Standards Codification
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Weighted average interest rate of long-term debt outstanding calculated at point in time.

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Long-term borrowings (Details 4) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 16, 2020	Oct. 30, 2020	Jun. 15, 2020	Dec. 31, 2019	Jul. 26, 2019
Long-term debt outstanding [Line Items]
Long-term debt	$ 2,213,130				$ 2,243,107
Montana-Dakota Utilities Co. [Member]
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	49.00%
Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	53.00%
Intermountain Gas Company
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	51.00%
Centennial Energy Holdings, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	31.00%
WBI Energy Transmission, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization	41.00%
Senior Notes
Long-term debt outstanding [Line Items]
Long-term debt	$ 1,950,000				$ 1,850,000
Weighted average interest rate	4.40%
Senior Notes \| Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Long-term debt			$ 25,000	$ 50,000
Weighted average interest rate			3.34%	3.66%
Ratio of total debt to total capitalization as specified in debt covenants			65.00%	65.00%
Senior Notes \| WBI Energy Transmission, Inc.
Long-term debt outstanding [Line Items]
Long-term debt	$ 195,000	$ 25,000
Weighted average interest rate		3.26%
Ratio of total debt to total capitalization as specified in debt covenants	55.00%
Long-term private shelf agreement issuance capacity						$ 300,000
Remaining capacity under uncommitted private shelf agreement	$ 105,000
Revolving credit facility \| Montana-Dakota Utilities Co. [Member]
Long-term debt outstanding [Line Items]
Ratio of funded debt to total capitalization as specified in debt covenants	65.00%
Revolving credit facility \| Cascade Natural Gas [Member]
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Revolving credit facility \| Intermountain Gas Company
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Revolving credit facility \| Centennial Energy Holdings, Inc.
Long-term debt outstanding [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%