Represents the actual payment as a percentage of the target amount.

No definition available.

Name:	ottr_ActualPaymentPercentageOfTargetAmount
Namespace Prefix:	ottr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of shares authorized for actual payment.

+ References

No definition available.

+ Details

Name:	ottr_NumberOfSharesAuthorizedForActualPayment
Namespace Prefix:	ottr_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of trading days used to set the average beginning and ending share prices for the change in total shareholder value over the performance measurement period and to establish the grant-date fair value of the performance share awards.

+ References

No definition available.

+ Details

Name:	ottr_NumberOfTradingDays
Namespace Prefix:	ottr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period specified for average adjusted return, used to determine the achievement of a target.

+ References

No definition available.

+ Details

Name:	ottr_PeriodSpecifiedForAverageAdjustedReturn
Namespace Prefix:	ottr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

No definition available.

+ Details

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

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	srt_TitleOfIndividualAxis=srt_ExecutiveOfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=ottr_StockPerformanceAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Note 16 - Subsequent Events (Details Textual) - The 2014 Stock Incentive Plan [Member] - USD ($) $ in Millions			12 Months Ended
	Feb. 12, 2020	Feb. 13, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized			1,900,000
Executive Officer [Member] \| Stock Performance Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		55,600
Period Specified for Average Adjusted Return			3 years
Number of Trading Days		20 days
Number of Shares Authorized for Actual Payment		83,400
Executive Officer [Member] \| Stock Performance Awards [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		27,800
Subsequent Event [Member] \| Stock Performance Awards [Member]
Period Specified for Average Adjusted Return	3 years
Number of Trading Days	20 days
Subsequent Event [Member] \| Executive Officer [Member] \| Stock Performance Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Grant Date Fair Value	$ 3.3
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	55,000
Number of Shares Authorized for Actual Payment	82,500
Subsequent Event [Member] \| Executive Officer [Member] \| Stock Performance Awards [Member] \| Minimum [Member]
Actual Payment, Percentage of Target Amount	0.00%
Subsequent Event [Member] \| Executive Officer [Member] \| Stock Performance Awards [Member] \| Maximum [Member]
Actual Payment, Percentage of Target Amount	150.00%
Subsequent Event [Member] \| Executive Officer [Member] \| Stock Performance Awards [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,500
Subsequent Event [Member] \| Executive Officer [Member] \| Stock Performance Awards [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,500