Note 14 - Income Tax Expense - Continuing Operations - Effective Income Tax Rate (Details) (Parentheticals)	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Composite Federal and State Statutory Rate	39.00%	39.00%	39.00%	39.00%

Represents the effective income tax rate reconciliation, at federal and state statutory income tax rate.

No definition available.

Name:	ottr_EffectiveIncomeTaxRateReconciliationAtFederalAndStateStatutoryIncomeTaxRate
Namespace Prefix:	ottr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration