Debt (Details Textuals) (USD $) In Thousands, unless otherwise specified	9 Months Ended					9 Months Ended					9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Line of Credit [Member]	Dec. 31, 2011 Line of Credit [Member]	Sep. 30, 2012 Senior Notes [Member]	Mar. 31, 2006 Senior Notes [Member]	Sep. 30, 2012 Senior Notes Due 2013 [Member]	Sep. 30, 2012 Senior Notes Due 2016 [Member]	Sep. 30, 2012 Senior Notes Due 2018 [Member]	Sep. 30, 2012 Industrial development revenue bonds [Member]	Dec. 31, 2011 Industrial development revenue bonds [Member]	Sep. 30, 2012 Uncommitted Line of Credit [Member]	Dec. 31, 2011 Uncommitted Line of Credit [Member]
Debt (Textuals)
Borrowing limit of short term uncommitted line of credit		$ 50,000									$ 106,319
Weighted average interest rate on outstanding borrowings											2.41%	4.23%
Maturity time of short term uncommitted lines											less than one year
Amount available under credit facility		167,000									31,535
Line of Credit Facility, Current Borrowing Capacity		500,000
Credit facility maturity period		5 years
Increase in borrowing limits under credit facility agreement		250,000
Weighted average interest rate on credit facility borrowings outstanding		1.33%	1.49%
Line of credit facility description of variable rate basis		LIBOR
Issuance of Senior Notes, Principal amount					300,000
Senior Notes due						75,000	175,000	50,000
Cash flow hedge				$ 200,000
Effective interest rate before cash flow hedge				5.50%
Effective interest rate after cash flow hedge				5.36%
Debt, Weighted Average Interest Rate					5.50%				0.75%	0.77%
Maturity of bonds issued									Jun. 01, 2017
Bond maturity period									20 years
Debt Instrument, Covenant Compliance	As of September 30, 2012, the Company was in compliance with the financial covenants in its debt agreements.