Debt (Details Textuals) (USD $) In Thousands, unless otherwise specified	6 Months Ended		6 Months Ended				6 Months Ended			6 Months Ended
	Jun. 30, 2013	Mar. 31, 2006 Senior Notes [Member]	Jun. 30, 2013 Senior Notes Due 2016 [Member]	Jun. 30, 2013 Senior Notes Due 2018 [Member]	Jun. 30, 2013 Industrial development revenue bonds [Member]	Dec. 31, 2012 Industrial development revenue bonds [Member]	Jun. 30, 2013 Revolving Credit Facility [Member]	Dec. 31, 2012 Revolving Credit Facility [Member]	Jun. 30, 2013 Swing Line Sub-Facility [Member]	Jun. 30, 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										$ 109,613
Weighted average interest rate on outstanding borrowings										1.84%	2.81%
Maturity time of short term uncommitted lines										less than one year
Amount available under credit facility							125,000			45,201
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		50,000
Weighted average interest rate on credit facility borrowings outstanding							1.31%	1.33%
Line of credit facility description of variable rate basis							LIBOR
Issuance of Senior Notes, Principal amount		300,000
Notional Amount Of Senior Notes Interest Rate Cash Flow Hedge Derivatives		200,000
Effective interest rate before cash flow hedge		5.50%
Effective interest rate after cash flow hedge		5.36%
Repayments of Long-term Debt	75,000
Senior Notes Maturity Schedule			$ 175,000	$ 50,000
Debt, Weighted Average Interest Rate		5.50%			0.43%	0.49%
Maturity of bonds issued					Jun. 01, 2017
Bond maturity period					20 years
Debt Instrument, Covenant Compliance	As of June 30, 2013, the Company was in compliance with the financial covenants in its debt agreements.