Note 11 - Pension Plan and Other Postretirement Benefits - Measurement Dates (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Net Periodic Pension Cost	Jan. 01, 2019	Jan. 01, 2018
End of Year Benefit Obligations	January 1, 2019 projected to December 31, 2019	January 1, 2018 projected to December 31, 2018
Market Value of Assets	Dec. 31, 2019	Dec. 31, 2018
Other Postretirement Benefits Plan [Member]
Net Periodic Postretirement Benefit Cost	Jan. 01, 2019	Jan. 01, 2018
End of Year Benefit Obligations	January 1, 2019 projected to December 31, 2019	January 1, 2018 projected to December 31, 2018

Represents the measurement date for defined benefit plan benefit obligations.

No definition available.

Name:	ottr_DefinedBenefitPlanBenefitObligationsMeasurementDate
Namespace Prefix:	ottr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	ottr_DefinedBenefitPlanFairValueOfPlanAssetsMeasurementDate
Namespace Prefix:	ottr_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	ottr_DefinedBenefitPlanNetBenefitCostsMeasurementDate
Namespace Prefix:	ottr_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	ottr_DefinedBenefitPlanNetBenefitCostsMeasurementDateInformation
Namespace Prefix:	ottr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	ottr_DefinedBenefitPlanmNetPostretirementBenefitCostsMeasurementDate
Namespace Prefix:	ottr_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: