Name:	ottr_AssetRetirementCostsCapitalizedRollForward
Namespace Prefix:	ottr_
Data Type:	xbrli:stringItemType
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Represents the amount of gain on settlement of an asset retirement obligation deferred under regulatory accounting.

No definition available.

Name:	ottr_AssetRetirementObligationGainOnSettlement
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the amount of asset retirement obligation settlements attributable to accumulated depreciation.

No definition available.

Name:	ottr_AssetRetirementObligationSettlementsAccumulatedDepreciation
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the amount of asset retirement obligation settlements during the period attributable to an asset retirement obligation.

No definition available.

Name:	ottr_AssetRetirementObligationSettlementsAssetRetirementObligation
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the amount of asset retirement obligation settlements retired during the period attributable to original capitalized asset retirement cost.

No definition available.

Name:	ottr_AssetRetirementObligationSettlementsRetiredOriginalCapitalizedAssetRetirementCost
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
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Represents the amount of asset retirement obligation settlements during the period attributable to settlement cost.

No definition available.

Name:	ottr_AssetRetirementObligationSettlementsSettlementCost
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the amount of accumulated depreciation related to capitalized asset retirement costs as of a specified date.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsAccumulatedDepreciation
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents the amount of change in accumulated depreciation during the period related to capitalized asset retirement costs, which change is attributable to depreciation expense.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsAccumulatedDepreciationDepreciationExpense
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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Represents the effect of liabilities incurred during the period on the accumulated depreciation related to capitalized asset retirement costs.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsAccumulatedDepreciationLiabilitiesIncurred
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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Represents the amount of change in accumulated depreciation during the period related to capitalized asset retirement costs, which change is attributable to liabilities settled.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsAccumulatedDepreciationLiabilitiesSettled
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the amount of change in accumulated depreciation during the period related to capitalized asset retirement costs, which change is attributable to revisions in cash flow estimates.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsAccumulatedDepreciationRevisionOfEstimate
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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Represents the amount of liabilities incurred during the period capitalized as asset retirement costs.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsLiabilitiesIncurred
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the amount of adjustment to capitalized asset retirement costs during the period attributable to liabilities settled.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsLiabilitiesSettled
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Represents the amount of adjustment to capitalized asset retirement costs during the period attributable to revision of cash flow estimates.

No definition available.

Name:	ottr_CapitalizedCostsAssetRetirementCostsRevisionOfEstimate
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

Name:	us-gaap_AssetRetirementObligationAccretionExpense
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Data Type:	xbrli:monetaryItemType
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Amount of asset retirement obligations incurred during the period.

Name:	us-gaap_AssetRetirementObligationLiabilitiesIncurred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 35
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Name:	us-gaap_AssetRetirementObligationRevisionOfEstimate
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Data Type:	xbrli:monetaryItemType
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	us-gaap_AssetRetirementObligationRollForwardAnalysisRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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The amount of asset retirement costs previously incurred and capitalized separately from the capitalized amount of the associated long-lived assets.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447