Debt (Details Textuals) (USD $) In Thousands, unless otherwise specified	3 Months Ended				3 Months Ended					3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Line of Credit [Member]	Dec. 31, 2012 Line of Credit [Member]	Mar. 31, 2006 Senior Notes [Member]	Mar. 31, 2013 Senior Notes Due 2016 [Member]	Mar. 31, 2013 Senior Notes Due 2018 [Member]	Mar. 31, 2013 Senior Notes - Repayment in 2013 [Member]	Mar. 31, 2013 Industrial development revenue bonds [Member]	Dec. 31, 2012 Industrial development revenue bonds [Member]	Mar. 31, 2013 Uncommitted Line of Credit [Member]	Dec. 31, 2012 Uncommitted Line of Credit [Member]
Debt (Textuals)
Borrowing limit of short term uncommitted line of credit		$ 50,000								$ 110,006
Weighted average interest rate on outstanding borrowings										2.61%	2.81%
Maturity time of short term uncommitted lines										less than one year
Amount available under credit facility		118,000								55,923
Credit facility maturity period		5 years
Line of Credit Facility, Current Borrowing Capacity		500,000
Increase in borrowing limits under credit facility agreement		250,000
Weighted average interest rate on credit facility borrowings outstanding		1.34%	1.33%
Line of credit facility description of variable rate basis		LIBOR
Issuance of Senior Notes, Principal amount				300,000
Effective interest rate before cash flow hedge				5.50%
Effective interest rate after cash flow hedge				5.36%
Senior Notes Maturity Schedule					175,000	50,000
Repayments of Long-term Debt							75,000
Debt, Weighted Average Interest Rate				5.50%				0.39%	0.49%
Notional Amount Of Senior Notes Interest Rate Cash Flow Hedge Derivatives				$ 200,000
Maturity of bonds issued								Jun. 01, 2017
Bond maturity period								20 years
Debt Instrument, Covenant Compliance	As of March 31, 2013, the Company was in compliance with the financial covenants in its debt agreements.