Long-Term Debt and Commitments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term debt, gross	$ 85,500,000	$ 85,500,000
Less: Current maturities	77,260,000	76,977,000
Long-term debt	8,200,000	8,478,000
Short-term Borrowings [Abstract]
Uncommitted short-term bank credit lines, maximum borrowing capacity	191,200,000
Uncommitted short-term bank credit lines, amount outstanding	7,226,000	7,111,000
Series 2006 Senior Notes Due 2013 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	50,000,000	50,000,000
Interest rate on debt instrument (in hundredths)	5.53%
Maturity date	Oct. 05, 2013
Floating Rate Series 2006 A Senior Notes Due 2013 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	25,000,000	25,000,000
Maturity date	Oct. 05, 2013
Variable/Fixed Rate Industrial Development Revenue Bonds Series 1999 Due 2014 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	8,200,000	8,200,000
Maturity date	Nov. 01, 2014
Installment Obligations [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,400,000	1,400,000
Other Borrowings [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 900,000	$ 900,000