Costs incurred that are not recoverable from the customer.

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- Definition

Public Utilities Rate Base

+ References

No definition available.

+ Details

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- Definition

Requested Rate Increase For Delivery Service

+ References

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+ Details

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- Definition

Requested Rate Increase For Production

+ References

No definition available.

+ Details

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- Definition

Amount of cash outflow to customers for refund payments of revenue that was previously reported as deferred or unearned revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response))
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

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- Definition

Amount of public utility's approved rate increase (decrease) by regulatory agency.

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

+ References

No definition available.

+ Details

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- Definition

Percentage of public utility's requested debt to capital, including debt and equity.

+ References

No definition available.

+ Details

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- Definition

Percentage of public utility's requested equity to capital, including debt and equity.

+ References

No definition available.

+ Details

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- Definition

Amount of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

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- Definition

Percentage of public utility's requested return on equity.

+ References

No definition available.

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Regulatory Matters (Details) - USD ($) $ in Millions	3 Months Ended
Regulatory Matters (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016
Montana Natural Gas Rate Filing [Member]
Public Utilities, General Disclosures [Line Items]
Requested rate increase	$ 9.4		$ 10.9
Requested rate increase for delivery service			7.4
Requested rate increase for production			$ 3.5
Requested return on equity, percentage			10.35%
Public utilities rate base			$ 432.1
Requested debt capital structure, percentage			53.00%
Requested equity capital structure, percentage			47.00%
Interim rate increase			$ 5.6
Hydro Transaction [Member] \| Revenue Subject to Refund [Member]
Public Utilities, General Disclosures [Line Items]
Approved rate decrease, amount		$ 1.2
Dave Gates Generating Station [Member]
Public Utilities, General Disclosures [Line Items]
Deferred revenue, refund payments				$ 27.3
Disallowed Expenses [Member]
Public Utilities, General Disclosures [Line Items]
Disallowed costs		$ 12.4