Business Segments (Details) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Business Segments [Line Items]
Operating Revenue	$ 226.9	$ 224.1	$ 689.0	$ 668.9
Fuel and Purchased Power Expense	74.8	79.0	229.8	233.1
Operating and Maintenance Expense	90.5	89.8	276.3	262.9
Depreciation Expense	22.7	20.0	67.1	59.8
Operating Income (Loss)	38.9	35.3	115.8	113.1
Interest Expense	(10.9)	(9.7)	(32.6)	(28.1)
Equity Earnings in ATC	4.7	4.5	13.7	13.4
Other Income (Expense)	0.5	0.6	2.3	3.8
Income (Loss) Before Non-Controlling Interest and Income Taxes	33.2	30.7	99.2	102.2
Income Tax Expense (Benefit)	12.7	11.2	24.7	40.5
Net Income (Loss)	20.5	19.5	74.5	61.7
Less: Non-Controlling Interest in Subsidiaries	0	(0.1)	(0.2)	(0.3)
Net Income (Loss) Attributable to ALLETE	20.5	19.6	74.7	62.0
Total Assets	2,754.4	2,579.1	2,754.4	2,579.1	2,609.1
Property, Plant and Equipment - Net	1,902.1	1,742.6	1,902.1	1,742.6	1,805.6
Accumulated Depreciation	1,079.0	1,022.2	1,079.0	1,022.2
Capital Additions			143.5	175.5
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	207.4	204.8	632.2	615.0
Fuel and Purchased Power Expense	74.8	79.0	229.8	233.1
Operating and Maintenance Expense	70.4	70.2	218.8	209.3
Depreciation Expense	21.4	18.9	63.5	56.6
Operating Income (Loss)	40.8	36.7	120.1	116.0
Interest Expense	(9.2)	(8.0)	(26.9)	(23.3)
Equity Earnings in ATC	4.7	4.5	13.7	13.4
Other Income (Expense)	0.6	1.3	1.8	3.6
Income (Loss) Before Non-Controlling Interest and Income Taxes	36.9	34.5	108.7	109.7
Income Tax Expense (Benefit)	13.1	12.4	28.2	44.5
Net Income (Loss)	23.8	22.1	80.5	65.2
Less: Non-Controlling Interest in Subsidiaries	0	0	0	0
Net Income (Loss) Attributable to ALLETE	23.8	22.1	80.5	65.2
Total Assets	2,436.0	2,299.7	2,436.0	2,299.7
Property, Plant and Equipment - Net	1,847.1	1,698.1	1,847.1	1,698.1
Accumulated Depreciation	1,028.6	973.2	1,028.6	973.2
Capital Additions			128.4	174.3
Investments and Other [Member]
Business Segments [Line Items]
Operating Revenue	19.5	19.3	56.8	53.9
Fuel and Purchased Power Expense	0	0	0	0
Operating and Maintenance Expense	20.1	19.6	57.5	53.6
Depreciation Expense	1.3	1.1	3.6	3.2
Operating Income (Loss)	(1.9)	(1.4)	(4.3)	(2.9)
Interest Expense	(1.7)	(1.7)	(5.7)	(4.8)
Equity Earnings in ATC	0	0	0	0
Other Income (Expense)	(0.1)	(0.7)	0.5	0.2
Income (Loss) Before Non-Controlling Interest and Income Taxes	(3.7)	(3.8)	(9.5)	(7.5)
Income Tax Expense (Benefit)	(0.4)	(1.2)	(3.5)	(4.0)
Net Income (Loss)	(3.3)	(2.6)	(6.0)	(3.5)
Less: Non-Controlling Interest in Subsidiaries	0	(0.1)	(0.2)	(0.3)
Net Income (Loss) Attributable to ALLETE	(3.3)	(2.5)	(5.8)	(3.2)
Total Assets	318.4	279.4	318.4	279.4
Property, Plant and Equipment - Net	55.0	44.5	55.0	44.5
Accumulated Depreciation	50.4	49.0	50.4	49.0
Capital Additions			$ 15.1	$ 1.2

The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.14)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 12

-Paragraph 5

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 14

-Article 5

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 12

-Paragraph 5

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.