Share-Based Payments (Detail Textuals) $ in Millions	9 Months Ended
Sep. 30, 2016 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock-based compensation \| $	$ 5.1
Weighted-average period of amortization	2 years 4 months 24 days
Stock Performance Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Targeted aggregate common shares award	81,500
Stock Performance Awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target amount as actual payment	0.00%
Stock Performance Awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate common shares award	122,250
Percentage of target amount as actual payment	150.00%
Stock Performance Awards \| Executive Officers
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Targeted aggregate common shares award total shareholder return component	54,333
Targeted aggregate common shares award return on equity component	27,167
Period specified for average adjusted return	3 years

Represents the percentage of target amount as a actual payment.

No definition available.

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

- Definition

Represents period specified for average adjusted return

+ References

No definition available.

+ Details

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

- 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=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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://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=6415400&loc=d3e5070-113901

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
Data Type:	na
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