Regulatory Matters (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended						12 Months Ended		12 Months Ended		3 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 South Dakota Rate Case [Member]	Dec. 31, 2011 Wind Generation [Member] watts	Dec. 31, 2011 Montana General Rate Case [Member]	Dec. 31, 2012 Montana Distribution System Infrastructure Project [Member]	Dec. 31, 2011 Montana Distribution System Infrastructure Project [Member] Scenario, Forecast [Member]	Dec. 31, 2011 Montana Distribution System Infrastructure Project [Member] Scenario, Forecast [Member] Deferred Project Costs [Member]	Dec. 31, 2012 Montana Distribution System Infrastructure Project [Member] Scenario, Plan [Member]	Dec. 31, 2011 Montana Distribution System Infrastructure Project [Member] Scenario, Plan [Member]	Dec. 31, 2010 Dave Gates Generating Station [Member] watts	Dec. 31, 2011 Dave Gates Generating Station [Member]	Dec. 31, 2012 Mountain States Transmission [Member]	Dec. 31, 2011 Mountain States Transmission [Member]	Dec. 31, 2011 Mountain States Transmission [Member] Southbound [Member] watts	Dec. 31, 2011 Mountain States Transmission [Member] Northbound [Member] watts
Regulatory Assets [Line Items]
Megawatts of capacity				40							150				1,500	1,100
Total project costs incurred to date												$ 183
Estimated construction costs				86							202
Deferred revenue												11.0
Percentage of revenues based on FERC												20.00%
Percentage of revenue based on MPSC												80.00%
Rate of return on equity for base rate			10.90%	7.40%	10.25%
Capitalized structure percentage of equity			56.00%	48.00%	48.00%
Base rate, natural gas distribution			67.5
Natural gas distribution revenue, approved rate increase			1.8
Estimated environmental remediation costs			1.4
Requested natural gas distribution revenue increase			4.1
Original authorized return on equity for base rate					10.25%
Revised return on equity for base rate					10.00%
Electric, original revenue requirement increase					7.7
Electric revised revenue requirement increase					6.4
Electric reduction of rates settled					0.7
Electric revenue requirement increase					7.0
Natural gas base rate decrease					1.0
Capitalized structure, percentage of Debt				52.00%	52.00%
Long term debt cost percentage				5.00%	5.76%
Incremental operating and maintenance costs approved for deferral, maximum amount						16.9
Estimated incremental project costs									12.0	72.0			7.9	0.2
Projected capital expenditure									18.2	253.0
Deferred costs								4.9
Payments for capital improvements							15.2
Computed return on equity for base rate				10.00%
Comtemplated strategic partner, ownership percentage, maximum														50.00%
Capitalized costs relating ro preliminary survey and investigation	$ 21.8	$ 19.0												$ 20.9