Business Segments (Details) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016 USD ($) a	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) a	Dec. 31, 2015 USD ($)		Dec. 31, 2014 USD ($)
Business Segments [Line Items]
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments										We present three reportable segments: Regulated Operations, ALLETE Clean Energy, and U.S. Water Services. We measure performance of our operations through budgeting and monitoring of contributions to consolidated net income by each business segment.
Number of Reportable Segments										3
Operating Revenue		$ 341.5	$ 349.6	$ 314.8	$ 333.8	$ 380.6	$ 462.5	$ 323.3	$ 320.0	$ 1,339.7	$ 1,486.4		$ 1,136.8
Net Income (Loss) Attributable to ALLETE		44.3	$ 40.3	$ 24.8	$ 45.9	18.3	$ 60.4	$ 22.5	$ 39.9	155.3	141.1		124.8
Depreciation and Amortization										195.8	170.0		135.7
Operating Expenses – Other										(10.3)	(36.3)		0.0
Interest Expense										70.3	64.9		54.8
Equity Earnings in ATC										18.5	16.3		19.6
Income Tax Expense (Benefit)										19.8	25.3		36.7
Assets	[1]	4,906.4				4,894.5				4,906.4	4,894.5
Capital Expenditures										$ 247.8	251.8
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments										3
Operating Revenue										$ 1,000.7	991.2		1,003.5
Net Income (Loss) Attributable to ALLETE										135.5	131.6	[2]	123.0
Depreciation and Amortization										154.3	135.1		118.0
Interest Expense										52.1	53.9	[2]	49.2
Equity Earnings in ATC										18.5	16.3		19.6
Income Tax Expense (Benefit)										5.9	24.4		39.0
Assets	[1]	3,853.4				3,853.1				3,853.4	3,853.1
Capital Expenditures										121.8	224.4
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue										80.5	262.1	[3]	33.2
Net Income (Loss) Attributable to ALLETE										13.4	29.9		3.3
Depreciation and Amortization										22.3	18.7		10.1
Operating Expenses – Other	[4]									(3.3)	0.0		0.0
Interest Expense										5.8	3.3		0.8
Income Tax Expense (Benefit)										8.1	21.0		2.3
Assets	[1]	566.0				501.5				566.0	501.5
Capital Expenditures										106.9	8.6
ALLETE Clean Energy [Member] | Wind Construction Project [Member]
Business Segments [Line Items]
Operating Revenue											197.7
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue										137.5	119.8		0.0
Net Income (Loss) Attributable to ALLETE										1.5	0.9		0.0
Depreciation and Amortization										8.9	7.3		0.0
Interest Expense										1.7	1.4		0.0
Income Tax Expense (Benefit)										1.4	0.9		0.0
Assets		$ 264.1				258.3				264.1	258.3
Capital Expenditures										$ 3.7	2.9
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments										2
Land in Minnesota (Acres) | a		5,000								5,000
Operating Revenue										$ 121.0	113.3		100.1
Net Income (Loss) Attributable to ALLETE	[2]									4.9	(21.3)		(1.5)
Depreciation and Amortization										10.3	8.9		7.6
Operating Expenses – Other	[4]									13.6	(36.3)		0.0
Interest Expense										14.5	8.6	[2]	7.1
Income Tax Expense (Benefit)										4.4	(21.0)		(4.6)
Assets		$ 222.9				$ 281.6				222.9	281.6
Capital Expenditures										15.4	15.9
Eliminations [Member]
Business Segments [Line Items]
Interest Expense										$ (3.8)	$ (2.3)	[2]	$ (2.3)

[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 reclassified to conform to the current presentation. (See Note 1. Operations and Significant Accounting Policies.)

[2] During 2015, an intercompany loan agreement was entered into and interest expense was allocated to certain subsidiaries. The amounts are eliminated in consolidation.

[3] Includes the construction and sale of a wind energy facility by ALLETE Clean Energy to Montana-Dakota Utilities for $197.7 million in 2015.

[4] See Note 1. Operations and Significant Accounting Policies.