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Note 8 - Leases - Operating Lease Obligation (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Operating Lease Obligations, beginning balance	$ 20,369
Non-cash Acquisition of Right-of-Use Assets	6,318
Lease Adjustments/Modifications	(116)
Lease Obligation Payments	(5,371)
Interest Component of Lease Obligation Payment	1,129
Operating Lease Obligations, ending balance	22,329
OTP [Member]
Operating Lease Obligations, beginning balance	3,609
Non-cash Acquisition of Right-of-Use Assets	758
Lease Adjustments/Modifications	71
Lease Obligation Payments	(1,316)
Interest Component of Lease Obligation Payment	170
Operating Lease Obligations, ending balance	3,292
Nonelectric Companies [Member]
Operating Lease Obligations, beginning balance	16,760
Non-cash Acquisition of Right-of-Use Assets	5,560
Lease Adjustments/Modifications	(187)
Lease Obligation Payments	(4,055)
Interest Component of Lease Obligation Payment	959
Operating Lease Obligations, ending balance	$ 19,037

X

- Definition

Amount of operating lease interest expense.

+ References

No definition available.

+ Details

Name:	ottr_OperatingLeaseInterest
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The amount of change in operating lease liability related to the lease modifications.

+ References

No definition available.

+ Details

Name:	ottr_OperatingLeaseLeaseModifications
Namespace Prefix:	ottr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

Name:	us-gaap_OperatingLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of cash outflow from operating lease, excluding payments to bring another asset to condition and location necessary for its intended use.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918643-209977

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (g)(1)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918686-209980

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of increase in right-of-use asset obtained in exchange for operating lease liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 55
-Paragraph 53
-URI http://asc.fasb.org/extlink&oid=77888399&loc=SL77918982-209971

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (g)(2)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918686-209980

+ Details

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

X

- Details

Name:	srt_ConsolidatedEntitiesAxis=ottr_OTPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_ConsolidatedEntitiesAxis=ottr_NonelectricCompaniesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: