Name:	ottr_JointlyOwnedUtilityPlantOwnershipAmountOrDeficitOfConstructionWorkInProgress
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

Name:	us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The total period end amount of plant in service and construction in progress net of accumulated depreciation for the utility's share of jointly owned electricity generation plant.

Name:	us-gaap_JointlyOwnedUtilityPlantNetOwnershipAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

Name:	us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The proportionate share of interest (percent) in a jointly owned utility plant.

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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Namespace Prefix:
Data Type:	na
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Name:	us-gaap_JointlyOwnedUtilityPlantAxis=ottr_CoyoteStationMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_JointlyOwnedUtilityPlantAxis=ottr_BigStoneSouthEllendaleMultiValueProjectMember
Namespace Prefix:
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