Capacity of a generating facility, unit, or project in megawatts.

No definition available.

Name:	ale_GeneratingCapacityMw
Namespace Prefix:	ale_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

- Definition

Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ References

No definition available.

+ Details

Name:	ale_LongtermContractforPurchaseofElectricPowerTermofContractYears
Namespace Prefix:	ale_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerAmountOfLongTermDebtOrLeaseObligationOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The allocable portion of interest included in charges under a long-term contract to purchase electric power.

+ References

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerInterestIncludedInContractCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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_LongTermContractForPurchaseOfElectricPowerLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ References

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of operating expense for purchased power procured under arrangements extending beyond one year or the normal operating cycle, if longer, by regulated operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(b))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

+ Details

Name:	us-gaap_UtilitiesOperatingExpensePurchasedPowerUnderLongTermContracts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) MW	Jun. 30, 2016 USD ($)	Jul. 28, 2017 MW
Square Butte [Member] \| Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Total Debt Outstanding \| $	$ 311.6
Cost of Power Purchased \| $	40.7	$ 37.7
Pro Rata Share of PPA Counterparty Interest Expense \| $	$ 4.7	$ 4.8
Square Butte [Member] \| Square Butte PPA [Member] \| Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)	455
Output Entitlement	50.00%
Minnkota Power [Member] \| Square Butte Coal-fired Unit [Member] \| Minnkota Power PSA [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	28.00%	28.00%
Tenaska [Member] \| Tenaska PPA [Member]
Power Purchase Agreements [Line Items]
Contract Term (Years)	20 years
Wind Turbine Generators [Member] \| Tenaska [Member] \| Tenaska PPA [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)	250
Subsequent Event [Member] \| Integrated Resource Plan [Member] \| MPUC [Member] \| Minimum [Member] \| Natural Gas-Fired [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)			525
Subsequent Event [Member] \| Integrated Resource Plan [Member] \| MPUC [Member] \| Maximum [Member] \| Natural Gas-Fired [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW)			550