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Short-Term and Long-Term Borrowings - Narrative (Details)	Mar. 31, 2021
Maximum
Debt Instrument [Line Items]
Debt to total capitalization ratio	60.00%
Minimum
Debt Instrument [Line Items]
Interest and dividend coverage ratio	1.50

X

- Definition

Represents the required ratio of debt to total capitalization.

+ References

No definition available.

+ Details

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

X

- Definition

Represents the required interest and dividend coverage ratio.

+ References

No definition available.

+ Details

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Namespace Prefix:	ottr_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

X

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

X

- Details

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

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: