Regulatory matters and revenues subject to refund (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	0 Months Ended	1 Months Ended
Sep. 30, 2011 NDPSC [Member]	Feb. 08, 2011 MTPSC [Member]	Aug. 31, 2010 MTPSC [Member]
Regulated Operations, Schedule of Regulatory Matters [Line Items]
Total electric rate increase requested			$ 5.5
Percent above current rates requested (in hundredths)			13.00%
Total amount of requested interim electric rate increase			3.1
Percentage of interim electric rate increase requested (in hundredths)			7.40%
Amount of approved interim increase in annual electric rates		2.6
Approved percentage of interim electric rate increase (in hundredths)		6.28%
Estimated project cost of the natural gas turbine and associated facilities	$ 85.6

The amount of the approved interim increase in annual electric rates

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The approved percentage of interim electric rate increase.

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Estimated project cost of the natural gas turbine and associated facilities.

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The percentage of the annual increase in electric rate over current rates requested by Montana-Dakota.

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The percentage of requested increase in interim electric rates over the current rates.

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The total amount of the requested interim electric rate increase.

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The total amount of the annual increase in electric rate requested by Montana-Dakota.

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