Business Segments Business Segment (Details) $ in Millions		3 Months Ended												12 Months Ended
		Dec. 31, 2018 USD ($) a	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Revenues		$ 448.3	$ 348.0	$ 344.1	$ 358.2	$ 337.9	$ 362.5	$ 353.3	$ 365.6	$ 341.5	$ 349.6	$ 314.8	$ 333.8	$ 1,498.6	$ 1,419.3	$ 1,339.7
Net Income (Loss) Attributable to Parent		61.1	$ 30.7	$ 31.3	$ 51.0	41.4	$ 44.9	$ 36.9	$ 49.0	$ 44.3	$ 40.3	$ 24.8	$ 45.9	174.1	172.2	155.3
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes	[1]													0.0	13.0	0.0
Depreciation, Depletion and Amortization														205.6	177.5	195.8
Operating Expenses – Other														(2.0)	(0.7)	(10.3)
Interest Expense														67.9	67.8	70.3
Equity Earnings in ATC														17.5	22.5	18.5
Income Tax Expense (Benefit)														(15.5)	14.7	19.8
Assets		5,165.0				5,080.0								5,165.0	5,080.0
Property, Plant and Equipment, Additions														$ 318.6	266.5
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments														3
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes															0.0
Depreciation, Depletion and Amortization														$ 158.0	132.6	154.3
Interest Expense	[2]													60.2	57.0	52.1
Equity Earnings in ATC														17.5	22.5	18.5
Income Tax Expense (Benefit)														(15.5)	27.2	5.9
Property, Plant and Equipment, Additions														211.9	177.1
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenues														1,059.5	1,063.8	1,000.7
Net Income (Loss) Attributable to Parent	[3],[4]													131.0	128.4	135.5
Assets		$ 3,952.5				3,886.6								$ 3,952.5	3,886.6
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments														2
Land in Minnesota (Acres) | a		4,000												4,000
Revenues														$ 107.1	123.2	121.0
Net Income (Loss) Attributable to Parent	[3],[4],[5]													6.2	(8.4)	4.9
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes															(19.8)
Depreciation, Depletion and Amortization														13.0	11.7	10.3
Operating Expenses – Other	[6]													(2.0)	(0.7)	(13.6)
Interest Expense	[2]													7.3	10.3	14.5
Income Tax Expense (Benefit)														0.0	9.5	4.4
Assets		$ 310.1				300.5								310.1	300.5
Property, Plant and Equipment, Additions														12.0	28.9
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Interest Expense	[2]													(4.7)	(5.3)	(3.8)
ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenues														159.9	80.5	80.5
Net Income (Loss) Attributable to Parent	[3],[4],[7]													33.7	41.5	13.4
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes															23.6
Depreciation, Depletion and Amortization														24.4	23.4	22.3
Operating Expenses – Other	[6]													0.0	0.0	3.3
Interest Expense	[2]													3.6	4.2	5.8
Income Tax Expense (Benefit)														(1.0)	(14.2)	8.1
Assets		606.6				600.5								606.6	600.5
Property, Plant and Equipment, Additions														89.7	56.1
U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenues														172.1	151.8	137.5
Net Income (Loss) Attributable to Parent	[3],[4]													3.2	10.7	1.5
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes															9.2
Depreciation, Depletion and Amortization														10.2	9.8	8.9
Interest Expense	[2]													1.5	1.6	1.7
Income Tax Expense (Benefit)														1.0	(7.8)	1.4
Assets		$ 295.8				$ 292.4								295.8	292.4
Property, Plant and Equipment, Additions														5.0	4.4
Residential [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													139.7	127.4	127.9
Commercial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													147.9	139.8	139.5
Municipal [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													54.9	57.9	67.6
Industrial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													469.5	470.5	416.0
Other Power Suppliers [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													170.3	161.8	175.1
CIP Financial Incentive [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													3.0	5.5	7.5
Other [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													74.2	100.9	67.1
Long-term PSA [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													55.2	56.9	58.2
Sale of Wind Energy Facility [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Sale of Wind Energy Facility	[8]													81.1	0.0	0.0
ALLETE Clean Energy Other [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue, Other Than Customer Revenue	[8]													23.6	23.6	22.3
Point-in-Time [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													100.3	95.8	93.9
Contract [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													38.3	36.2	30.4
Capital Project [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													33.5	19.8	13.2
Long-term Contract [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													85.5	89.3	73.7
Corporate & Other - Other [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													$ 21.6	33.9	47.3
MPUC [Member] | Retail Customers [Member] | Renewable Cost Recovery Rider [Member] | Electric Rates [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions															7.9	8.8
Income Tax Expense [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent															0.0
Income Tax Expense [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Income Tax Expense (Benefit)															14.0	(15.0)
Income Tax Expense [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Income Tax Expense (Benefit)															$ (7.9)	$ 8.8

[1] For the year ended December 31, 2017, the federal deferred income tax benefit is due to the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.

[2] Includes interest expense resulting from intercompany loan agreements and allocated to certain subsidiaries. The amounts are eliminated in consolidation

[3] Includes interest expense resulting from intercompany loan agreements and allocated to certain subsidiaries. The amounts are eliminated in consolidation.

[4] Net income in 2017 included a favorable impact of $13.0 million after-tax due to the remeasurement of deferred income tax assets and liabilities resulting from the TCJA, which consisted of a $23.6 million after-tax benefit for ALLETE Clean Energy, a $9.2 million after-tax benefit for U.S. Water Services and a $19.8 million after-tax expense for Corporate and Other. The TCJA did not have an impact on net income for our Regulated Operations as the remeasurement of deferred income tax assets and liabilities primarily resulted in the recording of regulatory assets and liabilities. (See Note 1. Operations and Significant Accounting Policies and Note 4. Regulatory Matters.)

[5] Net income in 2017 included a $7.9 million after-tax favorable impact for the regulatory outcome of the MPUC’s modification of tits November 2016 order on the allocation of North Dakota investment tax credits. Net income in 2016 included an $8.8 million after-tax adverse impact for the regulatory outcome of the November 2016 order.

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

[7] Net income in 2018 includes the recognition of profit for the sale of a wind energy facility to Montana-Dakota Utilities.

[8] With the adoption of new revenue recognition guidance, the Company has enhanced the presentation of business segment Operating Revenue. (See Note 1. Operations and Significant Accounting Policies.)