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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
	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: