Long-Term Debt (Details) (USD $)	6 Months Ended
	Feb. 28, 2013 item	Feb. 29, 2012 item
Long-term debt
Outstanding amount	$ 128,600,000	$ 134,400,000
Long-term line of credit | CoBank, ACB
Long-term debt
Maximum borrowing capacity	60,800,000
Additional borrowing under line of credit, which can be utilized for either short-term or long-term borrowing purposes	50,000,000
Amount outstanding	64,000,000	69,800,000
Additional line of credit outstanding		9,000,000
Unused borrowing capacity which can be utilized for short-term borrowings	46,800,000	41,000,000
Outstanding amount	0	0
Term Loans from Insurance Companies
Long-term debt
Outstanding amount	50,000,000	50,000,000
Pollution Control and Industrial Development Revenue Bonds
Long-term debt
Outstanding amount	$ 78,600,000	$ 84,400,000
Number of bond issuances	4	5