Debt (Details Textuals) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2013	Mar. 31, 2006	Dec. 31, 2014
Debt Instrument [Line Items]
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 (Textuals)
Repayments of Long-term Debt		$ 75.0
Debt Instrument, Covenant Compliance	As of March 31, 2015 the Company was in compliance with the financial covenants in its debt agreements.
Senior Notes [Member]
Debt (Textuals)
Issuance of Senior Notes, Principal amount			300.0
Notional Amount Of Senior Notes Interest Rate Cash Flow Hedge Derivatives			200.0
Debt, Weighted Average Interest Rate			5.50%
Effective interest rate before cash flow hedge			5.50%
Effective interest rate after cash flow hedge			5.36%
Senior Notes Due 2016 [Member]
Debt (Textuals)
Senior Notes Maturity Schedule	175.0
Senior Notes Due 2018 [Member]
Debt (Textuals)
Senior Notes Maturity Schedule	50.0
Industrial development revenue bonds [Member]
Debt (Textuals)
Debt, Weighted Average Interest Rate	0.21%			0.27%
Maturity of bonds issued	Jun. 01, 2017
Bond maturity period	20 years
Revolving Credit Facility [Member]
Debt (Textuals)
Line of Credit Facility, Current Borrowing Capacity	520.0			500.0
Increase in borrowing limits under credit facility agreement	250.0
Weighted average interest rate on credit facility borrowings outstanding	1.69%			1.69%
Line of credit facility description of variable rate basis	LIBOR
Amount available under credit facility	225.0
Swing Line Sub-Facility [Member]
Debt (Textuals)
Increase in borrowing limits under credit facility agreement	50.0
Uncommitted Line of Credit [Member]
Debt (Textuals)
Borrowing limit of short term uncommitted line of credit	122.3
Weighted average interest rate on outstanding borrowings	3.02%			2.96%
Maturity time of short term uncommitted lines	less than one year
Amount available under credit facility	78.3