Short-Term and Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Short-Term and Long-Term Debt [Abstract]
Short-Term Debt Outstanding	$ 67.4	$ 67.4	$ 6.5
Short-Term Debt, Amount Expected to be Refinanced		60
Debt Instrument [Line Items]
Long-Term Debt Outstanding	808.4	808.4	857.9
Proceeds from Issuance of First Mortgage Bonds	160.0
Required Indebtedness to Total Capital Ratio	0.65 to 1.00
Actual Indebtedness to Total Capital Ratio	0.43 to 1.00	0.43 to 1.00
ALLETE LOC Jan2014 [Member]
Debt Instrument [Line Items]
Line of Credit, Amount Outstanding	14.0	14.0
Line of Credit, Maximum Borrowing Capacity	150.0	150.0
ALLETE First Mortgage Bonds 3.20% Due July2026 [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of First Mortgage Bonds	75.0
Interest Rate	3.20%	3.20%
ALLETE First Mortgage Bonds 4.08% Due July2042 [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of First Mortgage Bonds	85.0
Interest Rate	4.08%	4.08%
Industrial Development Revenue Bonds 6.5% Due 2025 [Member]
Debt Instrument [Line Items]
Interest Rate	6.50%	6.50%
Amount of Redeemed Bonds	$ 6.0