Financial Instruments (Narrative) (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified			9 Months Ended		9 Months Ended					9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Private Placement, Notes [Member]	Dec. 31, 2011 Private Placement, Notes [Member]	Sep. 30, 2012 Credit Agreement [Member]	Dec. 31, 2011 Credit Agreement [Member]	Jul. 16, 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]	Sep. 30, 2012 Convertible Notes [Member]	Dec. 31, 2011 Convertible Notes [Member]	Mar. 31, 2006 Convertible Notes [Member]	Sep. 30, 2012 Warrants [Member]
Debt Instrument [Line Items]
Carrying amount of debt	$ 322,195,000	$ 374,388,000	$ 150,000,000	$ 150,000,000	$ 134,000,000	$ 187,000,000				$ 28,052,000	$ 27,228,000
Debt issued										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			174,600,000							28,300,000
Amount of credit facility					390,000,000				460,000,000
Amount of credit facility outstanding					134,000,000
Additional amount that can be borrowed on facility					256,000,000
Notional amount								105,000,000
Fixed interest rate in swap							0.46%	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	1.06
Interest coverage ratio	13.56
Date when holders may convert										Feb. 15, 2013
Shares per $1,000 of principal										23.2078
Minimum per share distribution value allowing conversion										15.00%
Percent of product which allows conversion										103.00%
Conversion price										$ 43.09
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
Date warrants can be exercised										Sep. 15, 2013
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													$ 49.2