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.)