Long-Term Debt and Credit Agreements (Details) (USD $)	12 Months Ended								12 Months Ended		1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Credit Agreement [Member]	Dec. 31, 2011 Credit Agreement [Member]	Dec. 31, 2010 Credit Agreement [Member]	Dec. 31, 2010 Unsecured Senior Notes [Member]	Dec. 31, 2003 Unsecured Senior Notes [Member]	Dec. 31, 2012 Industrial Revenue Bonds [Member]	Dec. 31, 2011 Industrial Revenue Bonds [Member]	Dec. 31, 2003 6.08% Senior Unsecured Notes [Member] Unsecured Senior Notes [Member]	Dec. 31, 2003 6.81% Senior Unsecured Notes [Member] Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity on line of credit						$ 180,000,000
Remaining borrowing capacity under line of credit				116,100,000
Average interest rate on debt instruments				3.81%	3.25%
Face amount of debt instruments								100,000,000			65,000,000	35,000,000
Stated interest rate of debt instruments (as a percentage)											6.08%	6.81%
Contractual term of debt instruments (in years)											7 years	10 years
Senior note maturity paid with credit agreement							65,000,000
Long-term debt	92,814,000	74,030,000		57,814,000	37,800,000				0	1,230,000
Amounts repaid on long-term debt	27,258,000	305,000	65,380,000						1,200,000
Amount outstanding on letters of credit	6,000,000
Maturities of long-term debt under the loan agreements, 2013	35,000,000
Maturities of long-term debt under the loan agreements, 2014	0
Maturities of long-term debt under the loan agreements, 2015	$ 57,814,000