Note 5 - Common Shares and Earnings Per Share (Details Textual) - Distribution Agreement With KeyBanc [Member]	Nov. 08, 2019 USD ($)
Sale of Stock, Maximum Aggregate Sales Price	$ 75,000,000
Sale of Stock, Commission Percent	2.00%

The maximum aggregate sales price for a sale of stock.

No definition available.

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