Debt (Details Textuals) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Mar. 31, 2006
Debt Instrument [Line Items]
Borrowing capacity under credit facility	$ 500,000
Debt (Textuals)
Borrowing limit of short term uncommitted line of credit	100,372
Weighted average interest rate on outstanding borrowings	2.70%	3.01%
Maturity time of short term uncommitted lines	Less than one year
Short term uncommitted lines of credit matures less than one year	63,261
Credit facility maturity period	5 years
New line of credit borrowed to repay existing credit facility	330,000
Increase in borrowing limits under credit facility agreement	250,000
Revolving credit facility available under the swing line sub facility	50,000
Weighted average interest rate on credit facility borrowings outstanding	1.32%	2.71%
Line of credit facility description of variable rate basis	LIBOR
Amount available under credit facility	148,000
Issuance of Senior Notes, Principal amount			300,000
Weighted average Interest rate of senior notes			5.50%
Cash flow hedge	200,000
Reduced Interest rate	Reduction in effective interest rate by 14 basis points from 5.50 to 5.36 percent
Reduction in effective interest rate from cash flow hedge	0.14%
Effective interest rate before cash flow hedge	5.50%
Effective interest rate after cash flow hedge	5.36%
Bond maturity period	20 years
Maturity of bonds issued	2017
Weighted average interest rates on industrial revenue bonds	0.66%	0.57%
Debt instrument covenant compliance	Company was in compliance with the financial covenants in its debt agreements
Fees to creditors	1,864
Senior Notes Due 2013 [Member]
Debt Instrument [Line Items]
Senior Notes due	75,000
Senior Notes Due 2016 [Member]
Debt Instrument [Line Items]
Senior Notes due	175,000
Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Senior Notes due	$ 50,000