Short-Term and Long-Term Borrowings - Status of lines of credit (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Line Limit	$ 320,000
In Use	1,117
Restricted due to Outstanding Letters of Credit	1,869
Available	317,014	316,078
Otter Tail Corporation Credit Agreement
Line of Credit Facility [Line Items]
Line Limit	150,000
In Use
Restricted due to Outstanding Letters of Credit	680
Available	149,320	149,267
OTP Credit Agreement
Line of Credit Facility [Line Items]
Line Limit	170,000
In Use	1,117
Restricted due to Outstanding Letters of Credit	1,189
Available	$ 167,694	$ 166,811