Represents the required ratio of debt to total capitalization.

No definition available.

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

- Definition

Represents the required interest and dividend coverage ratio.

+ References

No definition available.

+ Details

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Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Increased Current Borrowing Capacity, Accordion Provision

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ottr_OTPCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ottr_The2019NotePurchaseAgreementMember
Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ottr_OtterTailPowerCompanyMember
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Short-Term and Long-Term Borrowings - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Line Limit	$ 340,000,000
Debt, weighted average interest rate	1.61%	3.20%
Minimum
Debt Instrument [Line Items]
Interest and dividend coverage ratio	1.50
Maximum
Debt Instrument [Line Items]
Debt to total capitalization ratio	60.00%
Benchmark rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	0.00%
Benchmark rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.00%
OTC Credit Agreement
Debt Instrument [Line Items]
Line Limit	$ 170,000,000
Line of credit facility, increased current borrowing capacity, accordion provision	290,000,000
OTC Credit Agreement \| Letter of Credit
Debt Instrument [Line Items]
Line Limit	$ 40,000,000
OTC Credit Agreement \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.50%
OTP Credit Agreement
Debt Instrument [Line Items]
Line Limit	$ 170,000,000
Line of credit facility, increased current borrowing capacity, accordion provision	250,000,000
OTP Credit Agreement \| Letter of Credit
Debt Instrument [Line Items]
Line Limit	$ 50,000,000
OTP Credit Agreement \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.25%
The 2019 Note Purchase Agreement \| OTP
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 75,000,000.0