Short-Term and Long-Term Borrowings (Detail Textuals) - USD ($)

9 Months Ended

Sep. 30, 2016

Oct. 31, 2016

Sep. 23, 2016

Debt Instrument [Line Items]

Line Limit

$ 320,000,000

Otter Tail Corporation Credit Agreement

Debt Instrument [Line Items]

Line Limit

$ 150,000,000

Otter Tail Corporation Credit Agreement | Subsequent Event

Debt Instrument [Line Items]

Line Limit

$ 130,000,000

2016 Note Purchase Agreement

Debt Instrument [Line Items]

Debt instrument description of prepayment

The 2016 Note Purchase Agreement states the Company may prepay all or any part of the 2026 Notes (in an amount not less than 10% of the aggregate principal amount of the 2026 Notes then outstanding in the case of a partial prepayment) at 100% of the principal amount prepaid, together with unpaid accrued interest and a make-whole amount; provided that if no default or event of default exists under the 2016 Note Purchase Agreement, any optional prepayment made by the Company of all of the 2026 Notes on or after September 15, 2026 will be made without any make-whole amount.

Priority debt maximum percentage of total capitalization

10.00%

Financial covenants of debt

The Company may not permit the ratio of its Interest-bearing Debt (as defined in the 2016 Note Purchase Agreement) to Total Capitalization (as defined in the 2016 Note Purchase Agreement) to be greater than 0.60 to 1.00, determined as of the end of each fiscal quarter, and may not permit the Interest and Dividend Coverage Ratio (as defined in the 2016 Note Purchase Agreement) to be less than 1.50 to 1.00 for any period of four consecutive fiscal quarters. The Company is also restricted from allowing its Priority Debt (as defined in the 2016 Note Purchase Agreement) to exceed 10% of Total Capitalization, determined as of the end of each fiscal quarter.

2016 Note Purchase Agreement | Minimum

Debt Instrument [Line Items]

Debt to total capitalization ratio

0.60

Interest and dividend coverage ratio

1.00

2016 Note Purchase Agreement | Maximum

Debt Instrument [Line Items]

Debt to total capitalization ratio

1.00

Interest and dividend coverage ratio

1.50