Operations and Significant Accounting Policies - Reclassifications (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended
	Dec. 31, 2014	Mar. 31, 2014
Consolidated Balance Sheet [Member] | Property, Plant and Equipment - Net to Goodwill and Intangible Assets - Net [Member]
Reclassifications [Line Items]
Reclassification Adjustment	$ 1.6
Consolidated Balance Sheet [Member] | Other Non-Current Assets to Goodwill and Intangible Assets - Net [Member]
Reclassifications [Line Items]
Reclassification Adjustment	3.2
Consolidated Balance Sheet [Member] | Total Assets [Member]
Reclassifications [Line Items]
Reclassification Adjustment	0
Consolidated Statement of Income [Member] | Operating and Maintenance Expenses to Cost of Sales [Member]
Reclassifications [Line Items]
Reclassification Adjustment		23.5
Consolidated Statement of Income [Member] | Operating and Maintenance Expenses to Transmission Services [Member]
Reclassifications [Line Items]
Reclassification Adjustment		10.8
Consolidated Statement of Income [Member] | Operating and Maintenance Expenses to Taxes Other than Income Taxes [Member]
Reclassifications [Line Items]
Reclassification Adjustment		11.2
Consolidated Statement of Income [Member] | Operating Income [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0
Consolidated Statement of Income [Member] | Net Income [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0
Consolidated Statement of Income [Member] | Net Income Attributable to ALLETE [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0
Consolidated Statement of Cash Flows [Member] | Changes in Regulatory and Other Non-Current Liabilities to Amortization of Power Purchase Agreements [Member]
Reclassifications [Line Items]
Reclassification Adjustment		2.3
Consolidated Statement of Cash Flows [Member] | Operating Activities [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0
Consolidated Statement of Cash Flows [Member] | Investing Activities [Member]
Reclassifications [Line Items]
Reclassification Adjustment		0
Consolidated Statement of Cash Flows [Member] | Financing Activities [Member]
Reclassifications [Line Items]
Reclassification Adjustment		$ 0