XML 138 R115.htm IDEA: XBRL DOCUMENT
v3.10.0.1

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.)

X
- Definition Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes + References No definition available. + Details
Name: | ale_DeferredFederalIncomeTaxExpenseBenefitRemeasurementofDeferredIncomeTaxes
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Non-cash Revenue Recognized From Amortization of PSAs Assumed Through Acquisitions + References No definition available. + Details
Name: | ale_RevenueOtherThanCustomerRevenue
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Sale of Wind Energy Facility + References No definition available. + Details
Name: | ale_SaleofWindEnergyFacility
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Area of land held. + References No definition available. + Details
Name: | us-gaap_AreaOfLand
Namespace Prefix: | us-gaap_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | instant

X
- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(11)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(12)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_Assets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
Name: | us-gaap_DepreciationDepletionAndAmortization
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(12)) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(10)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(f)) -URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 + Details
Name: | us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 20 -Section 45 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109238882&loc=d3e38679-109324 + Details
Name: | us-gaap_IncomeTaxExpenseBenefit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of the cost of borrowed funds accounted for as interest expense. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (210.5-03(11)) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.9) -URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260 + Details
Name: | us-gaap_InterestExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 1A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669619-108580 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(22)) -URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 1B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669625-108580 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(18)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(20)) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
Name: | us-gaap_NetIncomeLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The net amount of other operating income and expenses, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operations. + References No definition available. + Details
Name: | us-gaap_OtherOperatingIncomeExpenseNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentAdditions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045 + Details
Name: | us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 32 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 + Details
Name: | us-gaap_Revenues
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=115928272&loc=d3e28511-109314 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=115928272&loc=d3e28446-109314 + Details
Name: | us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ale_RegulatedOperationsMemberMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ale_RegulatedOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=srt_ConsolidationEliminationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ale_ALLETECleanEnergyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ale_U.S.WaterServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_ResidentialMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_CommercialMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_MunicipalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_IndustrialMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_OtherPowerSuppliersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_CIPFinancialIncentiveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_OtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_LongtermPSAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_SaleofWindEnergyFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_ALLETECleanEnergyOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_PointinTimeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_ContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_CapitalProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_LongtermContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_CorporateOtherOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=ale_MPUCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ale_CustomerClassAxis=ale_RetailCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_RenewableCostRecoveryRiderMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ale_RateTypeAxis=ale_ElectricRatesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=ale_IncomeTaxExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |