Financial Instruments (Narrative) (Details) (USD $) Share data in Millions, unless otherwise specified		3 Months Ended						0 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Private Placement, Notes [Member]	Mar. 31, 2013 Credit Agreement [Member]	Dec. 31, 2010 Credit Agreement [Member]	Mar. 31, 2013 Credit Agreement [Member] Interest Rate Swap [Member]	Jan. 16, 2013 Credit Agreement [Member] Interest Rate Swap [Member]	Jul. 16, 2010 Credit Agreement [Member] Interest Rate Swap [Member]	Mar. 15, 2013 Convertible Notes [Member]	Mar. 31, 2013 Convertible Notes [Member]
Debt Instrument [Line Items]
Debt issued		$ 150,000,000							$ 28,437,000
Interest rate		6.84%							2.25%
Maturity date		Oct. 25, 2017
Year of maturity			2018						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		170,600,000
Amount of credit facility			330,000,000	390,000,000
Amount of credit facility outstanding			176,000,000
Additional amount that can be borrowed on facility			154,000,000
Notional amount					105,000,000		105,000,000
Fixed interest rate in swap							2.04%
LIBOR spread						0.31%
Effective annual rate					3.415%
Maximum leverage ratio allowed	3.50
Minimum interest coverage ratio required	3.00
Leverage ratio	1.36
Interest coverage ratio	11.16
Percentage of principal redeemed								100.00%
Cash paid to redeem convertible senior notes								$ 28,400,000
Number of warrants sold									4.1
Strike price of warrants									52.25
Date warrants can be exercised									Jun. 15, 2013