Debt (Details Textuals) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended		9 Months Ended				9 Months Ended				9 Months Ended
	Mar. 31, 2013	Sep. 30, 2014	Mar. 31, 2006 Senior Notes [Member]	Sep. 30, 2014 Senior Notes Due 2016 [Member]	Sep. 30, 2014 Senior Notes Due 2018 [Member]	Sep. 30, 2014 Industrial development revenue bonds [Member]	Dec. 31, 2013 Industrial development revenue bonds [Member]	Sep. 30, 2014 Revolving Credit Facility [Member]	Jun. 30, 2014 Revolving Credit Facility [Member]	Dec. 31, 2013 Revolving Credit Facility [Member]	Sep. 30, 2014 Swing Line Sub-Facility [Member]	Sep. 30, 2014 Uncommitted Line of Credit [Member]	Dec. 31, 2013 Uncommitted Line of Credit [Member]
Debt (Textuals)
Borrowing limit of short term uncommitted line of credit												$ 124,096
Weighted average interest rate on outstanding borrowings												2.91%	3.24%
Maturity time of short term uncommitted lines												less than one year
Amount available under credit facility								204,987				41,337
Line of Credit Facility, Current Borrowing Capacity								520,000	500,000
Increase in borrowing limits under credit facility agreement								250,000			50,000
Weighted average interest rate on credit facility borrowings outstanding								1.48%		1.36%
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
Debt, Weighted Average Interest Rate			5.50%			0.28%	0.36%
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
Maturity of bonds issued						Jun. 01, 2017
Bond maturity period						20 years
Debt Instrument, Covenant Compliance		As of September 30, 2014 the Company was in compliance with the financial covenants in its debt agreements.