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v3.21.1

Short-Term and Long-Term Borrowings - Narrative (Details) | Mar. 31, 2021
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Maximum |
Debt Instrument [Line Items] |
Debt to total capitalization ratio | 60.00%
Minimum |
Debt Instrument [Line Items] |
Interest and dividend coverage ratio | 1.50

X
- Definition Represents the required ratio of debt to total capitalization. + References No definition available. + Details
Name: | ottr_DebtToTotalCapitalizationRatio
Namespace Prefix: | ottr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the required interest and dividend coverage ratio. + References No definition available. + Details
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Namespace Prefix: | ottr_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |