Regulatory matters and revenues subject to refund (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
NDPSC [Member]
Regulated Operations, Schedule of Regulatory Matters [Line Items]
Total electric rate increase requested	$ 15.4
Percent above current rates requested (in hundredths)	14.00%
Amount of approved interim increase in annual electric rates	7.6
Revised total annual electric rate increase requested	8.8
Revised percentage of increase requested (in hundredths)	7.70%
Reductions in rate increase sought by regulatory agency	8.3
Amount of total rate increase included in settlement agreement	7.6
The number of MW of a simple cycle natural gas turbine	88
Estimated project cost of the natural gas turbine and associated facilities	85.6
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%