Consolidated Balance Sheet - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Current Assets [Abstract]
Cash and Cash Equivalents		$ 107.2	$ 97.0
Accounts Receivable (Less Allowance of $1.8 and $1.0)		108.1	121.2
Inventories		107.4	117.1
Prepayments and Other		38.9	35.7
Total Current Assets		361.6	371.0
Property, Plant and Equipment – Net		3,644.5	3,669.1
Regulatory Assets		358.9	372.0
Investment in ATC		133.8	124.5
Other Investments		58.1	74.6
Goodwill and Intangible Assets – Net		211.4	215.2
Other Non-Current Assets		106.5	68.1
Total Assets	[1]	4,874.8	4,894.5
Current Liabilities [Abstract]
Accounts Payable		72.6	88.8
Accrued Taxes		36.7	44.0
Accrued Interest		14.8	18.6
Long-Term Debt Due Within One Year		186.6	35.7
Notes Payable		0.0	1.6
Other		93.6	86.1
Total Current Liabilities		404.3	274.8
Long-Term Debt		1,358.9	1,556.7
Deferred Income Taxes		582.1	579.8
Regulatory Liabilities		122.9	105.0
Defined Benefit Pension and Other Postretirement Benefit Plans		197.7	206.8
Other Non-Current Liabilities		335.9	349.0
Total Liabilities		3,001.8	3,072.1
Commitments, Guarantees and Contingencies (Note 13)
ALLETE's Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 49.5 and 49.1 Shares Outstanding		1,289.4	1,271.4
Accumulated Other Comprehensive Loss		(23.7)	(24.5)
Retained Earnings		607.3	573.3
Total ALLETE Equity		1,873.0	1,820.2
Non-Controlling Interest in Subsidiaries		0.0	2.2
Total Equity		1,873.0	1,822.4
Total Liabilities and Equity		$ 4,874.8	$ 4,894.5

[1] As a result of revised accounting guidance adopted in the first quarter of 2016, we reclassified unamortized debt issuance costs from Other Non-Current Assets to Long-Term Debt on the Consolidated Balance Sheet. Prior period segment assets have been revised to conform to the current presentation. (See Note 1. Operations and Significant Accounting Policies.)