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 maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ 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=109197908&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_AwardTypeAxis=ottr_StockPerformanceAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_ExecutiveOfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=ottr_The2014StockIncentivePlanMember
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:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
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:

Note 16 - Subsequent Events (Details Textual) - shares			12 Months Ended
Note 16 - Subsequent Events (Details Textual) - shares	Feb. 13, 2019	Feb. 05, 2018	Dec. 31, 2018
Stock Performance Awards [Member] \| Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized
Period Specified for Average Adjusted Return			3 years
The 2014 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized			1,900,000
The 2014 Stock Incentive Plan [Member] \| Stock Performance Awards [Member] \| Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		54,000
Number of Trading Days		20 days
Subsequent Event [Member] \| The 2014 Stock Incentive Plan [Member] \| Stock Performance Awards [Member]
Period Specified for Average Adjusted Return	3 years
Number of Trading Days	20 days
Subsequent Event [Member] \| The 2014 Stock Incentive Plan [Member] \| Stock Performance Awards [Member] \| Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	55,600
Actual Payment, Percentage of Target Amount	150.00%
Number of Shares Authorized for Actual Payment	83,400
Subsequent Event [Member] \| The 2014 Stock Incentive Plan [Member] \| Stock Performance Awards [Member] \| Executive Officer [Member] \| Minimum [Member]
Actual Payment, Percentage of Target Amount	0.00%
Subsequent Event [Member] \| The 2014 Stock Incentive Plan [Member] \| Stock Performance Awards [Member] \| Executive Officer [Member] \| Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,800
Subsequent Event [Member] \| The 2014 Stock Incentive Plan [Member] \| Stock Performance Awards [Member] \| Executive Officer [Member] \| Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	27,800