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Note 6 - Share-based Payments - Stock Incentive Awards Granted to Officers Under the 2014 Stock Incentive Plan (Details)	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Restricted Stock Units (RSUs) [Member] | The 2014 Stock Incentive Plan [Member] | February 3, 2020 [Member] | Management [Member]
Shares/units granted (in shares) | shares	3,000
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 54.0450
Shares/units granted, vesting date	25% per year through February 6, 2024
Restricted Stock Units (RSUs) [Member] | The 2014 Stock Incentive Plan [Member] | February 12, 2020 [Member] | Executive Officer [Member]
Shares/units granted (in shares) | shares	15,300
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 54.0607
Shares/units granted, vesting date	25% per year through February 6, 2024
Restricted Stock Units (RSUs) [Member] | The 2014 Stock Incentive Plan [Member] | April 20, 2020 [Member] | Non-executive Employees [Member]
Shares/units granted (in shares) | shares	14,975
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 40.18
Shares/units granted, vesting date	100% April 8, 2024
Performance Shares [Member] | The 2014 Stock Incentive Plan [Member] | February 12, 2020 [Member]
Shares/units granted (in shares) | shares	47,600
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 47.10
Shares/units granted, vesting date	December 31, 2022
Performance Shares [Member] | The 2014 Stock Incentive Plan Under Legacy Agreement [Member] | February 12, 2020 [Member]
Shares/units granted (in shares) | shares	7,400
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 52.20
Shares/units granted, vesting date	December 31, 2022
Restricted Stock [Member] | The 2014 Stock Incentive Plan [Member] | April 20, 2020 [Member] | Nonemployee Directors [Member]
Shares/units granted (in shares) | shares	17,400
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 44.85
Shares/units granted, vesting date	33% per year through April 8, 2023

X

- Definition

Represents the award vesting date under a share-based compensation arrangement.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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=120381028&loc=d3e5070-113901

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X

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued 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://fasb.org/us-gaap/role/ref/legacyRef
-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=120381028&loc=d3e5070-113901

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X

- Details

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- Details

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