Debt (Textuals) (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended	12 Months Ended
	May 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2011	Mar. 31, 2006
Debt Instruments [Line Items]
Borrowing capacity under credit facility		$ 500,000
Line of credit facility maturity period	3 years	5 years
New line of credit borrowed to repay existing credit facility					330,000
Increase in borrowing limits under credit facility agreement		250,000
Weighted average interest rate on credit facility borrowings outstanding		1.49%	2.71%
Line of credit facility description of variable rate basis		LIBOR
Amount available under credit facility		209,000
Fees to creditors		1,876	0	4,539
Issuance of Senior Notes, Principal amount						300,000
Cash flow hedge		200,000
Long-term Debt, by Maturity [Abstract]
2012		21,722
2013		76,054
2014		713
2015		630
2016		466,717
Thereafter		62,040
Discussion Of Senior Notes Interest Rate Cash Flow Hedge Effective Rate		This reduced the 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%
Interest Expense, Debt		26,002	27,520	23,796
Debt Instrument Maturity Period		20 years
Debt instrument covenant compliance		As of December 31, 2011, the Company was in compliance with the financial covenants in its debt agreements.
Uncommitted Line of Credit [Member]
Debt Instruments [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		101,530
Weighted average interest rate on outstanding borrowings		4.23%	3.01%
Borrowing capacity under credit facility		79,958
Uncommitted Line of Credit [Member] | Maximum [Member]
Debt Instruments [Line Items]
Line of credit facility maturity period		1 year
Swing Line Loan [Member]
Debt Instruments [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		50,000
Senior Notes [Member]
Debt Instruments [Line Items]
Weighted average Interest rate of senior notes						5.50%
Senior Notes Due 2013 [Member]
Debt Instruments [Line Items]
Senior Notes due		75,000
Senior Notes Due 2016 [Member]
Debt Instruments [Line Items]
Senior Notes due		175,000
Senior Notes Due 2018 [Member]
Debt Instruments [Line Items]
Senior Notes due		50,000
Industrial development revenue bonds [Member]
Debt Instruments [Line Items]
Weighted average Interest rate of senior notes		0.77%	0.57%
Long-term Debt, by Maturity [Abstract]
Interest Expense, Debt		$ 88	$ 72	$ 122
Maturity of bonds issued		2017