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Note 1 - Summary of Significant Accounting Policies - Supplemental Disclosure of Cash Flow Information (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Transactions Related to Capital Additions not Settled in Cash	$ 112,314	$ 15,893

X

- Definition

Represents the accounts payable outstanding related to capital additions as part of noncash Investing Activities

+ References

No definition available.

+ Details

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