Financial Instruments (Narrative) (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified			3 Months Ended		3 Months Ended						3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Private Placement, Notes [Member]	Dec. 31, 2011 Private Placement, Notes [Member]	Mar. 31, 2012 Credit Agreement [Member]	Dec. 31, 2011 Credit Agreement [Member]	Jul. 16, 2010 Credit Agreement [Member]	Jan. 17, 2012 Credit Agreement [Member] Interest Rate Swap [Member]	Jul. 16, 2010 Credit Agreement [Member] Interest Rate Swap [Member]	Dec. 31, 2006 Previous Line Of Credit [Member]	Mar. 31, 2012 Convertible Notes [Member]	Dec. 31, 2011 Convertible Notes [Member]	Mar. 31, 2006 Convertible Notes [Member]	Mar. 31, 2012 Senior Notes [Member]	Mar. 31, 2012 Warrants [Member]
Debt Instrument [Line Items]
Carrying amount of debt	$ 326,781,000	$ 374,388,000	$ 150,000,000	$ 150,000,000	$ 139,000,000	$ 187,000,000					$ 27,632,000	$ 27,228,000
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			174,300,000								27,500,000
Amount of credit facility							390,000,000			460,000,000
Amount of credit facility outstanding					139,000,000
Letters of credit outstanding					51,000,000
Additional amount that can be borrowed on facility					199,000,000
Notional amount									105,000,000
Fixed interest rate in swap								0.57%	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	1.31
Interest coverage ratio	10.97
Date when holders may convert											Feb. 15, 2013
Shares per $1,000 of principal											23.0467
Principal amount outstanding											28,400,000
Minimum per share distribution value allowing conversion											15.00%
Percent of product which allows conversion											103.00%
Conversion price											$ 43.39
Earliest redemption date											Mar. 15, 2013
Redemption 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
Incremental net shares will be included in future diluted earnings per share, maximum average common stock price														$ 43.39	$ 49.83