Note 7 - Share-based Payments (Details Textual) $ in Millions	6 Months Ended
Jun. 30, 2017 USD ($) shares
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized \| $	$ 5.5
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 146 days
Stock Performance Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	59,500
Stock Performance Awards [Member] \| Minimum [Member]
Percentage of Target Amount as Actual Payment	0.00%
Stock Performance Awards [Member] \| Maximum [Member]
Percentage of Target Amount as Actual Payment	150.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	89,250
Stock Performance Awards [Member] \| Executive Officer [Member]
Targeted Aggregate Common Shares Award Total Shareholder Return Component	39,667
Threshold Trading Days for Common Stock	20 days
Targeted Aggregate Common Shares Award Return on Equity Component	19,833
Period Specified for Average Adjusted Return	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	59,500

Represents the percentage of target amount as a actual payment.

No definition available.

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Namespace Prefix:	ottr_
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Balance Type:	na
Period Type:	duration

- Definition

Represents period specified for average adjusted return

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
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Balance Type:	na
Period Type:	duration

- Definition

This element represents targeted aggregate common shares award return on equity component.

+ References

No definition available.

+ Details

Name:	ottr_TargetedAggregateCommonSharesAwardReturnOnEquityComponent
Namespace Prefix:	ottr_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents targeted aggregate common shares award total shareholder return component.

+ References

No definition available.

+ Details

Name:	ottr_TargetedAggregateCommonSharesAwardTotalShareholderReturnComponent
Namespace Prefix:	ottr_
Data Type:	xbrli:sharesItemType
Balance Type:	na
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- Definition

The threshold trading days for common stock.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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Balance Type:	debit
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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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