Summary of Significant Accounting Policies - Supplemental disclosure of cash flow information (Details 6) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Noncash Investing Activities:
Transactions Related to Capital Additions not Settled in Cash	$ 11,552	$ 21,760

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