Business Segments Business Segment (Details) $ in Millions		3 Months Ended												12 Months Ended
		Dec. 31, 2019 USD ($) a	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	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, 2019 USD ($) a	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments														3	3
Revenues		$ 304.6	$ 288.3	$ 290.4	$ 357.2	$ 448.3	$ 348.0	$ 344.1	$ 358.2	$ 337.9	$ 362.5	$ 353.3	$ 365.6	$ 1,240.5	$ 1,498.6	$ 1,419.3
Net Income		49.7	$ 31.2	$ 34.2	$ 70.5	61.1	$ 30.7	$ 31.3	$ 51.0	$ 41.4	$ 44.9	$ 36.9	$ 49.0	185.6	174.1	172.2
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes	[1]													0.0	0.0	13.0
Depreciation, Depletion and Amortization														202.0	205.6	177.5
Operating Expenses – Other														0.0	(2.0)	(0.7)
Interest Expense														64.9	67.9	67.8
Equity Earnings														21.7	17.5	22.5
Income Tax Expense (Benefit)														(6.6)	(15.5)	14.7
Assets		5,482.8				5,165.0								5,482.8	5,165.0
Property, Plant and Equipment, Additions														626.6	318.6
Income Tax Expense (Benefit), Due to Sale of Subsidiary														$ 10.4
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments														3
Depreciation, Depletion and Amortization														$ 159.4	158.0	132.6
Interest Expense	[2]													58.9	60.2	57.0
Equity Earnings														21.7	17.5	22.5
Income Tax Expense (Benefit)	[3],[4]													(7.1)	(15.5)	27.2
Property, Plant and Equipment, Additions														230.9	211.9
Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenues														1,042.4	1,059.5	1,063.8
Net Income	[5],[6]													154.4	131.0	128.4
Assets		$ 4,130.8				3,952.5								$ 4,130.8	3,952.5
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments														2
Land in Minnesota (Acres) | a		4,000												4,000
Revenues														$ 105.1	107.1	123.2
Net Income	[5],[6],[7],[8]													19.9	6.2	(8.4)
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes																(19.8)
Depreciation, Depletion and Amortization														13.5	13.0	11.7
Operating Expenses – Other	[9]													0.0	(2.0)	(0.7)
Interest Expense	[2]													8.0	7.3	10.3
Income Tax Expense (Benefit)	[3],[4],[10]													12.8	0.0	9.5
Assets		$ 350.5				310.1								350.5	310.1
Property, Plant and Equipment, Additions														10.1	12.0
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Interest Expense	[2]													(5.0)	(4.7)	(5.3)
ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenues														59.6	159.9	80.5
Net Income	[5],[6],[11]													12.4	33.7	41.5
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes																23.6
Depreciation, Depletion and Amortization														26.8	24.4	23.4
Interest Expense	[2]													2.8	3.6	4.2
Income Tax Expense (Benefit)	[3],[4]													(11.9)	(1.0)	(14.2)
Assets		1,001.5				606.6								1,001.5	606.6
Property, Plant and Equipment, Additions														385.6	89.7
U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenues														33.4	172.1	151.8
Net Income	[5],[6]													(1.1)	3.2	10.7
Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes																9.2
Depreciation, Depletion and Amortization														2.3	10.2	9.8
Interest Expense	[2]													0.2	1.5	1.6
Income Tax Expense (Benefit)	[3],[4]													(0.4)	1.0	(7.8)
Assets	[12]	$ 0.0				$ 295.8								0.0	295.8
Property, Plant and Equipment, Additions	[12]													0.0	5.0
Gain on Sale of Affiliate, After-tax														13.2
Residential [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														139.6	139.7	127.4
Commercial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														145.7	147.9	139.8
Municipal [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														48.6	54.9	57.9
Industrial [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														476.4	469.5	470.5
Other Power Suppliers [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														153.7	170.3	161.8
CIP Financial Incentive [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														2.8	3.0	5.5
Other [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														75.6	74.2	100.9
Long-term PSA [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														48.0	55.2	56.9
Sale of Wind Energy Facility [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Sale of Wind Energy Facility														0.0	81.1	0.0
ALLETE Clean Energy Other [Member] | ALLETE Clean Energy [Member]
Segment Reporting Information [Line Items]
Revenue, Other Than Customer Revenue														11.6	23.6	23.6
Point-in-Time [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													19.0	100.3	95.8
Contract [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													9.2	38.3	36.2
Capital Project [Member] | U.S. Water Services [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[8]													5.2	33.5	19.8
Long-term Contract [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														82.8	85.5	89.3
Corporate & Other - Other [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax														$ 22.3	$ 21.6	33.9
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
Income Tax Expense [Member] | Regulated Operations [Member]
Segment Reporting Information [Line Items]
Net Income																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
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)

[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] In 2017, Regulated Operations includes $14.0 million of income tax expense related to North Dakota investment tax credits transferred to Corporate and Other and higher pre-tax income for the favorable impact for the regulatory outcome of the MPUC’s modification of its November 2016 order on the allocation of North Dakota investment tax credits. There was no impact to net income for Regulated Operations. Corporate and Other recorded an offsetting income tax benefit of $7.9 million in 2017. (See Note 4. Regulatory Matters.)

[4] Income tax expense in 2017 included an income tax benefit of $13.0 million due to the remeasurement of deferred income tax assets and liabilities resulting from the TCJA, which consisted of income tax benefits of $23.6 million for ALLETE Clean Energy and $9.2 million for U.S. Water Services as well as additional income tax expense of $19.8 million for Corporate and Other. The TCJA did not have an impact on income tax expense 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] Includes interest expense resulting from intercompany loan agreements and allocated to certain subsidiaries. The amounts are eliminated in consolidation. 

[6] 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.)

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

[8] On March 26, 2019, ALLETE sold U.S. Water Services. The Company recognized a gain on the sale of $13.2 million after-tax which is reflected in Corporate and Other. (See Note 1. Operations and Significant Accounting Policies.)

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

[10] On March 26, 2019, ALLETE sold U.S. Water Services. The Company recognized income tax expense of $10.4 million for the gain on sale of U.S. Water Services which is reflected in Corporate and Other. (See Note 1. Operations and Significant Accounting Policies.)

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

[12] On March 26, 2019, ALLETE sold U.S. Water Services. (See Note 1. Operations and Significant Accounting Policies.)