Debt (Textuals) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2006
Debt Instruments [Line Items]
Borrowing capacity under credit facility	$ 500,000
Amount available under credit facility	261,000
Increase in borrowing limits under credit facility agreement	250,000
Weighted average interest rate on credit facility borrowings outstanding	1.36%	1.33%
Line of credit facility description of variable rate basis	LIBOR
Fees to creditors	0	0	1,876
Issuance of Senior Notes, Principal amount				300,000
Cash flow hedge				200,000
Senior Notes due	75,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2015	974
2016	415,692
2017	13,392
2018	50,184
Long-term debt	480,242	617,534
Interest Expense, Debt	26,896	23,454	26,002
Debt Instrument Maturity Period	20 years
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.
Debt instrument covenant compliance	As of December 31, 2013, the Company was in compliance with the financial covenants in its debt agreements
Uncommitted Line of Credit [Member]
Debt Instruments [Line Items]
Borrowing capacity under credit facility	106,809
Weighted average interest rate on outstanding borrowings	3.24%	2.81%
Amount available under credit facility	63,747
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 2016
Debt Instruments [Line Items]
Senior Notes due	175,000
Senior Notes Due 2018
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	0.36%	0.49%
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest Expense, Debt	$ 96	$ 88	$ 88
Maturity of bonds issued	Jun. 01, 2017
Uncommitted Line of Credit [Member]
Debt Instruments [Line Items]
Line of credit facility expiration period	1 year