Financial Instruments (Narrative) (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended					12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Private Placement, Notes [Member]	Oct. 17, 2011 Private Placement, Notes [Member] Interest Rate Swap [Member]	Dec. 31, 2011 Credit Agreement [Member]	Jul. 16, 2010 Credit Agreement [Member]	Dec. 31, 2011 Credit Agreement [Member] Interest Rate Swap [Member]	Dec. 31, 2006 Previous Line Of Credit [Member]	Dec. 31, 2011 Convertible Debt [Member]	Mar. 31, 2006 Convertible Debt [Member]	Dec. 31, 2011 Senior Notes [Member]	Dec. 31, 2011 Warrants [Member]
Debt Instrument [Line Items]
Principal payments due in 2012	$ 1,300,000
Principal payments due in 2013	82,200,000
Principal payments due in 2014	3,800,000
Principal payments due in 2015	237,000,000
Principal payments due in 2016	0
Principal payments due thereafter	50,100,000
Interest paid	20,800,000	17,400,000	21,400,000
Debt issued				150,000,000						28,437,000	180,000,000
Interest rate				6.84%						2.25%
Maturity date				Oct. 25, 2017
Year of maturity						2015				2026
Payment required on October 25, 2013				50,000,000
Payment required on October 25, 2015				50,000,000
Fair value of long-term debt				172,700,000						27,400,000
Amount of credit facility							390,000,000		460,000,000
Amount of credit facility outstanding						187,000
Letters of credit outstanding						50,200,000
Additional amount that can be borrowed on facility						152,000,000
Notional amount								105,000,000
Fixed interest rate in swap					0.41%			2.04%
LIBOR spread					2.25%
Effective annual rate					4.29%
Maximum leverage ratio allowed	3.5
Minimum interest coverage ratio required	3
Leverage ratio	1.73
Interest coverage ratio	8.95
Date when holders may convert										2013-02-13
Shares per $1,000 of principal										23.0467
Principal amount outstanding										28,400,000
Percent of product which allows conversion										103.00%
Conversion price										$ 43.39
Earliest redemption date										Mar. 15, 2013
Repemption price, percent of principal										100.00%
Covenant, voting power										50.00%
Note hedge and warrant, net cost										14,700,000
Number of warrants sold										4.1
Strike price of warrants										52.25
Carrying amount of equity component										25,500,000
Reduction of debt component										134,600,000
Reduction of equity component										$ 5,200,000
Incremental net shares will be included in future diluted earnings per share, maximum average common stock price												$ 43.39	$ 49.83