Operations and Significant Accounting Policies (Details) (USD $)	12 Months Ended					0 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Feb. 10, 2015
Subsequent Events [Line Items]
Business Combination, Reason for Business Combination	The acquisitions below are consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth.
Investments and Other [Abstract]
Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement (Percent)	20.00%
Amortization of Power Purchase Agreements	$ 12,700,000		$ 0		$ 0
Trade Accounts Receivable [Abstract]
Billed	85,500,000		78,700,000
Unbilled	18,600,000		18,700,000
Less: Allowance for Doubtful Accounts	1,100,000		1,100,000
Total Accounts Receivable - Net	103,000,000		96,300,000
Inventories [Abstract]
Fuel	29,000,000	[1]	13,100,000	[1]
Materials and Supplies	51,500,000		46,200,000
Total Inventories	80,500,000		59,300,000
Impairement of Long-Lived Assets [Abstract]
Land Impairment	0		0		0
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause	16,300,000		23,000,000
Construction Costs for Development Project	48,200,000	[2]	0	[2]
Restricted Cash	2,700,000	[3]	0	[3]
Other	14,800,000		12,100,000
Total Prepayments and Other Current Assets	82,000,000		35,100,000
Other Non-Current Assets [Abstract]
Restricted Cash	5,300,000		5,400,000
Other Current Liabilities [Abstract]
Customer Deposits	19,700,000		26,000,000
Power Purchase Agreements	19,400,000	[4]	0	[4]
Construction Deposits Received for Development Project	54,300,000	[5]	0	[5]
Other	27,400,000		26,600,000
Total Other Current Liabilities	120,800,000		52,600,000
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	109,200,000	[6]	81,800,000	[6]	77,900,000
Power Purchase Agreements	110,700,000	[4]	0	[4]
Other	45,100,000		45,400,000
Total Other Non-Current Liabilities	265,000,000		127,200,000
Income Taxes [Abstract]
More Likely Than Not Definition	50.00%
Square Butte PPA [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Output Purchased (MW)	227.5
Square Butte Coal-fired Unit [Member] | Square Butte PPA [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Output Purchased (Percent)	50.00%
Municipal Customers [Member]
Customer Information [Line Items]
Number of Customers	16
Retail Customers [Member]
Customer Information [Line Items]
Number of Customers	144,000
Electric [Member]
Customer Information [Line Items]
Number of Customers	15,000
Natural Gas [Member]
Customer Information [Line Items]
Number of Customers	12,000
Water [Member]
Customer Information [Line Items]
Number of Customers	10,000
Subsequent Event [Member]
Subsequent Events [Line Items]
Business Acquisition, Effective Date of Acquisition						Feb. 10, 2015
Business Acquisition, Name of Acquired Entity						U.S. Water Services
Payments to Acquire Businesses, Gross						$ 168,000,000
Business Acquisition, Acquired Business Number of Customers						3,600
Acquired Business, Customer Retention						90.00%
Business Combination, Reason for Business Combination						The acquisition of U.S. Water Services is consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth.
Investments and Other [Member]
Investments and Other [Abstract]
Land in Minnesota (in acres)	5,000

[1] Fuel inventory was lower at December 31, 2013 primarily due to the timing of coal shipments.

[2] Construction Costs for Development Project acquired/incurred due to ALLETE Clean Energy’s acquisition on November 20, 2014 of a project to develop and construct a wind energy facility in 2015. (See Other Current Liabilities table and Note 7. Acquisitions.)

[3] Restricted Cash related to ALLETE Clean Energy’s wind energy facilities operating expense and capital distribution reserve requirements.

[4] Power Purchase Agreements were acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions in 2014. (See Note 7. Acquisitions.)

[5] Construction Deposits Received for Development Project are due to ALLETE Clean Energy’s project to develop and construct a wind energy facility in 2015. (See Prepayment and Other Current Assets table and Note 7. Acquisitions.)

[6] The increase in 2014 is primarily related to BNI Coal for coal mining expansion and ALLETE Clean Energy due to wind energy facilities acquisitions.