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Note 6 - Share-based Payments - Stock Incentive Awards Granted to Officers Under the 2014 Stock Incentive Plan (Details) (Parentheticals) - The 2014 Stock Incentive Plan [Member] | 6 Months Ended
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Jun. 30, 2020
Management [Member] | Restricted Stock Units (RSUs) [Member] | February 3, 2020 [Member] |
Shares/units granted, vesting percentage | 25.00%
Executive Officer [Member] | Restricted Stock Units (RSUs) [Member] | February 12, 2020 [Member] |
Shares/units granted, vesting percentage | 25.00%
Non-executive Employees [Member] | Restricted Stock Units (RSUs) [Member] | April 20, 2020 [Member] |
Shares/units granted, vesting percentage | 100.00%
Nonemployee Directors [Member] | Restricted Stock [Member] | April 20, 2020 [Member] |
Shares/units granted, vesting percentage | 33.00%

X
- Definition Annual percentage of vesting of award under share-based payment arrangement. + References No definition available. + Details
Name: | ottr_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsAnnualPercentage
Namespace Prefix: | ottr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_TitleOfIndividualAxis=srt_ManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=ottr_The2014StockIncentivePlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardDateAxis=ottr_February32020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=srt_ExecutiveOfficerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardDateAxis=ottr_February122020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=ottr_NonExecutiveEmployeesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardDateAxis=ottr_April202020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=ottr_NonemployeeDirectorsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |