Debt (Details Textuals) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Mar. 31, 2006
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 500,000
Debt (Textuals)
Maturity time of short term uncommitted lines	less than one year
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	1.24%	1.49%
Line of credit facility description of variable rate basis	LIBOR
Amount available under credit facility	185,000
Issuance of Senior Notes, Principal amount			300,000
Cash flow hedge	200,000
Discussion Of Senior Notes Interest Rate Cash Flow Hedge Effective Rate	This reduced the effective interest rate from 5.50 percent to 5.36 percent.
Effective interest rate before cash flow hedge	5.50%
Effective interest rate after cash flow hedge	5.36%
Bond maturity period	20 years
Debt Instrument, Covenant Compliance	As of June 30, 2012, the Company was in compliance with the financial covenants in its debt agreements.
Uncommitted Line of Credit [Member]
Debt (Textuals)
Borrowing limit of short term uncommitted line of credit	108,932
Weighted average interest rate on outstanding borrowings	2.59%	4.23%
Amount available under credit facility	47,394
Uncommitted Line of Credit [Member] | Maximum [Member]
Debt (Textuals)
Credit facility maturity period	1 year
Swing Line Loan [Member]
Debt (Textuals)
Borrowing limit of short term uncommitted line of credit	50,000
Senior Notes [Member]
Debt (Textuals)
Debt, Weighted Average Interest Rate			5.50%
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
Industrial development revenue bonds [Member]
Debt (Textuals)
Debt, Weighted Average Interest Rate	0.86%	0.77%
Maturity of bonds issued	2017