Represents the maximum per share differences between basic and diluted earnings per share in total or from continuing or discontinued operations.

No definition available.

Name:	ottr_MaximumPerShareDifferencesBetweenBasicAndDilutedEarningsPerShareInTotalOrFromContinuingOrDiscontinuedOperations
Namespace Prefix:	ottr_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

Reconciliation of Common Shareholders' Equity, Common Shares and Earnings Per Share (Detail Textuals) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	May 11, 2015
Stockholders Equity Note [Line Items]
Maximum per share differences between basic and diluted earnings per share in total or from continuing or discontinued operations	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Distribution Agreement \| J.P. Morgan Securities Inc. (JPMS)
Stockholders Equity Note [Line Items]
Agreement To Sell Shares Value					$ 75