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Note 5 - Common Shares and Earnings Per Share (Details Textual) - USD ($)	6 Months Ended	12 Months Ended			24 Months Ended	30 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2019	Nov. 08, 2019	May 03, 2018	Oct. 13, 2015
Maximum per Share Differences Between Basic and Diluted Earnings per Share in Total or from Continuing or Discontinued Operations		$ 0.016	$ 0.016	$ 0.016
The 2014 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,900,000	1,900,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1,010,110	1,010,110
The 1999 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,400,000	1,400,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	349,763	349,763
Share-based Compensation Arrangements by Share-based Payment Award, Percentage of Market Price at Eligible Employees Granted Options to Purchase Shares	85.00%	100.00%			100.00%	15.00%
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan		13,432		5,284
Open Market Share Purchases		3,672	7,757	4,202
Dividend Reinvestment and Share Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1,220,032	1,220,032
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan		53,339
Open Market Share Purchases		109,807	116,822
Shelf Registration, Shares								1,500,000
Prior Shelf Registration for Dividend Reinvestment and Share Purchase Plan [Member]
Stock Issued During Period, Shares for Cash Invested under Employee Stock Purchase Plan				97,698
Open Market Share Purchases			53,853	87,634
Shelf Registration, Shares									1,500,000
Distribution Agreement With KeyBanc [Member]
Sale of Stock, Maximum Aggregate Sales Price							$ 75,000,000
Sale of Stock, Commission Percent							2.00%

X

- Definition

Represents the maximum per share differences between basic and diluted earnings per share in total or from continuing or discontinued operations.

+ References

No definition available.

+ Details

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

X

- Definition

OTTR common shares acquired in the open market to provide shares for the plan.

+ References

No definition available.

+ Details

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

X

- Definition

The maximum commission percent for sale of stock.

+ References

No definition available.

+ Details

Name:	ottr_SaleOfStockCommissionPercent
Namespace Prefix:	ottr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The maximum aggregate sales price for a sale of stock.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Represents the percentage of the market price of common shares at the end of each six-month purchase period at which eligible employees may purchase shares.

+ References

No definition available.

+ Details

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

X

- Definition

Shelf registration for issuance of common shares.

+ References

No definition available.

+ Details

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

X

- Definition

Represents the number of shares for cash invested under employee stock purchase plan.

+ References

No definition available.

+ Details

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

X

- 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

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Balance Type:	na
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X

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ 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
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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X

- Details

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- Details

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X

- Details

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X

- Details

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X

- Details

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