Regulatory Matters (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011		Jun. 30, 2011		Jun. 30, 2010	May 24, 2011	Dec. 31, 2010		Nov. 02, 2010	Apr. 30, 2010	Dec. 31, 2009	Nov. 02, 2009	Dec. 31, 2011 Retail Utility Rates [Member]	Dec. 31, 2011 Water Rates [Member]	Dec. 31, 2011 Natural Gas Rates [Member]	Dec. 31, 2011 Electric Rates [Member]	Jun. 30, 2011 Deferred Fuel Costs [Member]	Dec. 31, 2010 Deferred Fuel Costs [Member]	Jun. 30, 2011 Pension Costs [Member]	Dec. 31, 2010 Pension Costs [Member]	Jun. 30, 2011 Environmental Restoration Costs [Member]	Dec. 31, 2010 Environmental Restoration Costs [Member]	Jun. 30, 2011 Deferred Income Tax Charges [Member]	Dec. 31, 2010 Deferred Income Tax Charges [Member]	Jun. 30, 2011 Asset Retirement Obligation Costs [Member]	Dec. 31, 2010 Asset Retirement Obligation Costs [Member]	Jun. 30, 2011 Rate Case Expenses [Member]	Dec. 31, 2010 Rate Case Expenses [Member]	Jun. 30, 2011 Loss on Reacquired Debt [Member]	Dec. 31, 2010 Loss on Reacquired Debt [Member]	Jun. 30, 2011 Other Regulatory Assets [Member]	Dec. 31, 2010 Other Regulatory Assets [Member]	Jun. 30, 2011 Medicare Part D [Member]	Dec. 31, 2010 Medicare Part D [Member]	Jun. 30, 2011 Plant Removal Obligations [Member]	Dec. 31, 2010 Plant Removal Obligations [Member]	Jun. 30, 2011 Other Regulatory Liabilities [Member]	Dec. 31, 2010 Other Regulatory Liabilities [Member]
Regulatory Matters [Abstract]
Amount of retail rate increase request to recover the investment costs												$ 81.0
Amount of estimated increase in revenues due to interim rates											48.5
Amount of amended retail rate increase										72.0
Requested percentage return on equity										11.25%
Requested percentage of capital structure related to equity										54.29%
Requested percentage of capital structure related to debt										45.71%
Amount of approved retail rate increase						53.5			53.5
Percentage return on common equity approved									10.38%
Percentage of approved equity ratio									54.29%
Amount of rate differential authorized for collection						3.2
Amount capitalized as part of the rate base as part of agreement for new retail rate						20.5
Amount of deferred tax liability reversed as part of agreement for retail rate increase			6.2
Number of municipalities included in non-affiliated municipal customers (in number of municipalities)	16		16
Number of private utilities included in non-affiliated municipal customers (in number of private utilities)	1		1
Start date of formula-based contract with City of Nashwauk	May 01, 2012		May 01, 2012
End date of formula-based contract with City of Nashwauk	Apr. 30, 2022		Apr. 30, 2022
Start date of formula-based contract with other municipal customers	Jul. 01, 2011		Jul. 01, 2011
End date of formula-based contract with other municipal customers	Jun. 30, 2019		Jun. 30, 2019
Length of termination notice required for non-affiliated municipal customers (in years)			3Y
Rates Subject To Change [Line Items]
Percentage of average rate increase (in hundredths)													2.40%	12.80%	2.50%	0.70%
Percentage increase in rates (in hundredths)													10.90%
Amount of estimated additional revenue that will be generated under rate increase							2.0
Amount of income tax benefit (expense) related to Medicare Part D	2.9		2.9		(4.0)
Regulatory asset related to Medicare Part D						5.0
Regulatory Assets [Line Items]
Current assets	15.0	[1]	15.0	[1]			20.6	[1]									15.0	20.6
Non-current assets	290.2		290.2				310.2												249.5	257.9	0	20.5	22.1	17.3	8.7	7.8	1.0	1.4	1.7	1.8	2.2	3.5	5.0	0
Total regulatory assets	305.2		305.2				330.8
Regulatory Liabilities [Line Items]
Non-Current liabilities	43.4		43.4				43.6																22.2	23.4											16.4	16.9	4.8	3.3
Total regulatory liabilities	$ 43.4		$ 43.4				$ 43.6

[1] Current regulatory assets are included in prepayments and other on the consolidated balance sheet.