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Pension Plan and Other Postretirement Benefits - Measurement Dates (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Pension Cost	Jan. 01, 2020	Jan. 01, 2019
End of Year Benefit Obligations	January 1, 2020 projected to December 31, 2020	January 1, 2019 projected to December 31, 2019
Market Value of Assets	Dec. 31, 2020	Dec. 31, 2019
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Postretirement Benefit Cost	Jan. 01, 2020	Jan. 01, 2019
End of Year Benefit Obligations	January 1, 2020 projected to December 31, 2020	January 1, 2019 projected to December 31, 2019

X

- Definition

Represents the measurement date for defined benefit plan benefit obligations.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
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Balance Type:	na
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X

- Definition

Represents the measurement date for the fair value of plan assets under a defined benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
Data Type:	xbrli:dateItemType
Balance Type:	na
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X

- Definition

Represents the measurement date of net benefit costs under a defined benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
Data Type:	xbrli:dateItemType
Balance Type:	na
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X

- Definition

Represents the measurement date information of net benefit costs under a defined benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
Data Type:	xbrli:stringItemType
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X

- Definition

Represents the measurement date of net post retirement benefit costs under a defined benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
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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

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X

- Details

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X

- Details

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