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v3.19.3.a.u2

Note 13 - Property, Plant and Equipment (Details Textual) | 12 Months Ended
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Dec. 31, 2019
Electric Plant [Member] | Minimum [Member] |
Property, Plant and Equipment, Useful Life | 5 years
Electric Plant [Member] | Maximum [Member] |
Property, Plant and Equipment, Useful Life | 82 years
Nonelectric Plant [Member] | Minimum [Member] |
Property, Plant and Equipment, Useful Life | 2 years
Nonelectric Plant [Member] | Maximum [Member] |
Property, Plant and Equipment, Useful Life | 40 years

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_PublicUtilityAxis=ottr_ElectricPlantMember
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: |

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

X
- Details
Name: | us-gaap_PublicUtilityAxis=ottr_NonelectricPlantMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |