Debt (Schedule of Long Term Debt Instruments) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Line of credit facility, expiration date	Oct. 29, 2018
Capital lease obligations	$ 123,200,000	$ 126,000,000
Total debt	894,200,000	816,800,000
Less current maturities	(52,949,000)	(51,376,000)
Long-term debt	841,335,000	765,425,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	128,600,000	53,000,000
Senior Notes [Member] | 4.875% Senior Notes [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	Mar. 15, 2023
Carrying value of debt	400,000,000	400,000,000
Convertible Debt [Member] | 2011 4.0% Notes [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	Jun. 15, 2014
Carrying value of debt	104,600,000	103,800,000
Convertible Debt [Member] | 2011 4.25% Notes [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	Dec. 15, 2014
Carrying value of debt	95,100,000	94,500,000
Convertible Debt [Member] | 2009 4.0% Notes [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	Jun. 15, 2014
Carrying value of debt	9,600,000	9,600,000
Convertible Debt [Member] | 2009 4.25% Notes [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	Dec. 15, 2014
Carrying value of debt	3,000,000	3,000,000
Capital Lease Obligations [Member] | Maximum [Member]
Debt Instrument [Line Items]
Lease expiration date	Mar. 30, 2020
Notes Payable [Member] | Equipment [Member]
Debt Instrument [Line Items]
Carrying value of debt	$ 30,100,000	$ 26,900,000
Notes Payable [Member] | Equipment [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, maturity date	May 01, 2018