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Credit facilities (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018
Line of credit facility and other debt [Abstract] |  |
Letters of credit at end of period | $ 0 |
Long-term debt | Commercial paper revolving credit agreement | Montana-Dakota Utilities Co. [Member] |  |
Line of credit facility and other debt [Abstract] |  |
Facility limit, maximum borrowing capacity | 175,000,000.0 |
Amount outstanding, end of period | 118,600,000 | $ 48,500,000
Letters of credit at end of period | $ 0 |
Expiration date | Dec. 19, 2024 |
Long-term debt | Commercial paper revolving credit agreement | Centennial Energy Holdings, Inc. |  |
Line of credit facility and other debt [Abstract] |  |
Facility limit, maximum borrowing capacity | $ 600,000,000.0 |
Amount outstanding, end of period | 104,300,000 | 289,600,000
Letters of credit at end of period | $ 0 |
Expiration date | Dec. 19, 2024 |
Long-term debt | Revolving credit facility | Montana-Dakota Utilities Co. [Member] |  |
Line of credit facility and other debt [Abstract] |  |
Amount outstanding, end of period | $ 0 | 48,500,000
Option to increase borrowings, maximum amount | 225,000,000.0 |
Long-term debt | Revolving credit facility | Cascade Natural Gas Corporation |  |
Line of credit facility and other debt [Abstract] |  |
Facility limit, maximum borrowing capacity | 100,000,000.0 |
Amount outstanding, end of period | 64,600,000 | 53,800,000
Letters of credit at end of period | $ 2,200,000 |
Expiration date | Jun. 07, 2024 |
Option to increase borrowings, maximum amount | $ 125,000,000.0 |
Long-term debt | Revolving credit facility | Intermountain Gas Company |  |
Line of credit facility and other debt [Abstract] |  |
Facility limit, maximum borrowing capacity | 85,000,000.0 |
Amount outstanding, end of period | 24,500,000 | $ 56,300,000
Letters of credit at end of period | $ 1,400,000 |
Expiration date | Jun. 07, 2024 |
Option to increase borrowings, maximum amount | $ 110,000,000.0 |
Long-term debt | Revolving credit facility | Centennial Energy Holdings, Inc. |  |
Line of credit facility and other debt [Abstract] |  |
Option to increase borrowings, maximum amount | $ 700,000,000.0 |

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- Definition The option to increase borrowing capacity to the stated maximum amount, at the option of the company, on stated conditions. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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- Definition Date the credit facility terminates, in CCYY-MM-DD format. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Details
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- Details
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