Note 1 - Summary of Significant Accounting Policies - Supplemental Disclosure of Cash Flow Information (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Transactions Related to Capital Additions not Settled in Cash	$ 16,193	$ 4,338

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

No definition available.

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