Financial Instruments (Narrative) (Details) (USD $)		9 Months Ended	0 Months Ended	9 Months Ended		9 Months Ended		0 Months Ended	9 Months Ended			0 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Private Placement, Notes [Member]	Sep. 23, 2013 Credit Agreement [Member]	Sep. 30, 2013 Credit Agreement [Member]	Dec. 31, 2010 Credit Agreement [Member]	Sep. 30, 2013 Credit Agreement [Member] Minimum [Member]	Sep. 30, 2013 Credit Agreement [Member] Maximum [Member]	Jul. 16, 2013 Credit Agreement [Member] Interest Rate Swap [Member]	Sep. 30, 2013 Credit Agreement [Member] Interest Rate Swap [Member]	May 20, 2013 Credit Agreement [Member] Interest Rate Swap [Member]	Jul. 16, 2010 Credit Agreement [Member] Interest Rate Swap [Member]	Mar. 15, 2013 Convertible Notes [Member]	Sep. 30, 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		167,700,000
Amount of credit facility				330,000,000	390,000,000
Amount of credit facility outstanding				152,000,000
Additional amount that can be borrowed on facility				178,000,000
Notional amount									105,000,000	110,000,000	105,000,000
Fixed interest rate in swap										1.414%	2.04%
LIBOR spread			1.375%			1.25%	1.875%	0.27%	0.18%
Effective annual rate									3.415%
Maximum leverage ratio allowed	3.50
Minimum interest coverage ratio required	3.00
Leverage ratio	1.77
Interest coverage ratio	8.30
Percentage of principal redeemed												100.00%
Cash paid to redeem convertible senior notes												28,400,000
Strike price of warrants													52.25
Final expiration date													Sep. 10, 2013
Notes hedge and warrant transactions, net cost													$ 14,700,000