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Commitments, Guarantees and Contingencies Minimum Payments for Certain Long-Term Commitments (Details)
$ in Millions

Dec. 31, 2017

USD ($)

MW

Jul. 28, 2017

MW

Natural Gas-Fired [Member] | MPUC [Member] | Resource Package [Member] | Jointly-Owned Electricity Generation Plant [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Minimum [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Natural Gas PPA [Member]

Minimum Annual Payments for Certain Long-Term Commitments [Line Items]

Generating Capacity Jointly Owned (MW) | MW

525

525

Natural Gas-Fired [Member] | MPUC [Member] | Resource Package [Member] | Jointly-Owned Electricity Generation Plant [Member] | Combined-Cycle Natural Gas-Fired Generating Facility [Member] | Maximum [Member] | Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] | Natural Gas PPA [Member]

Minimum Annual Payments for Certain Long-Term Commitments [Line Items]

Generating Capacity Jointly Owned (MW) | MW

550

550

Coal, Rail and Shipping Contracts [Member]

Minimum Annual Payments for Certain Long-Term Commitments [Line Items]

Minimum Payments in 2018

$ 29.0

Minimum Payments in 2019

1.8

Minimum Payments in 2020

0.0

Minimum Payments in 2021

0.0

Minimum Payments in 2022

0.0

Minimum Payments Thereafter

0.0

Leasing Agreements [Member]

Minimum Annual Payments for Certain Long-Term Commitments [Line Items]

Minimum Payments in 2018

14.2

Minimum Payments in 2019

12.8

Minimum Payments in 2020

9.5

Minimum Payments in 2021

7.3

Minimum Payments in 2022

6.1

Minimum Payments Thereafter

30.0

Long-Term Service Agreements [Member]

Minimum Annual Payments for Certain Long-Term Commitments [Line Items]

Minimum Payments in 2018

^[1]

11.0

Minimum Payments in 2019

^[1]

0.9

Minimum Payments in 2020

^[1]

0.0

Minimum Payments in 2021

^[1]

0.0

Minimum Payments in 2022

^[1]

1.0

Minimum Payments Thereafter

^[1]

11.0

PPAs [Member]

Minimum Annual Payments for Certain Long-Term Commitments [Line Items]

Minimum Payments in 2018

^[2]

104.5

Minimum Payments in 2019

^[2]

107.1

Minimum Payments in 2020

^[2]

115.0

Minimum Payments in 2021

^[2]

144.8

Minimum Payments in 2022

^[2]

144.7

Minimum Payments Thereafter

^[2]

$ 1,667.0

[1]

Consists of long-term service agreements for wind energy facilities.

[2]

Does not include the agreement with Manitoba Hydro expiring in 2022, as this contract is for surplus energy only; Oliver Wind I and Oliver Wind II, as Minnesota Power only pays for energy as it is delivered; and the agreement with Tenaska commencing in 2020 as it is subject to approval of the construction of a 525 MW to 550 MW combined-cycle natural gas-fired facility. (See Power Purchase Agreements.)

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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+ References

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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