Debt (Textuals) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2006
Debt Instruments [Line Items]
Borrowing capacity under credit facility	$ 500,000
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
Amount available under credit facility	200,000
Fees to creditors	0	1,876	0
Issuance of Senior Notes, Principal amount				300,000
Cash flow hedge	200,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2013	109,212
2014	1,351
2015	1,086
2016	476,662
2017	13,428
Thereafter	50,007
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.
Interest Expense, Debt	23,454	26,002	27,520
Debt Instrument Maturity Period	20 years
Debt instrument covenant compliance	As of December 31, 2012, the Company was in compliance with the financial covenants in its debt agreements
Uncommitted Line of Credit [Member]
Debt Instruments [Line Items]
Weighted average interest rate on outstanding borrowings	2.81%	4.23%
Borrowing capacity under credit facility	77,421
Line of Credit Facility, Maximum Borrowing Capacity	111,337
Line of Credit [Member]
Debt Instruments [Line Items]
Line of credit facility expiration period	5 years
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	0.49%	0.77%
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest Expense, Debt	$ 88	$ 88	$ 72
Maturity of bonds issued	Jun. 01, 2017
Uncommitted Line of Credit [Member]
Debt Instruments [Line Items]
Line of credit facility expiration period	1 year