Long-Term Debt (Details Textual) (USD $)	6 Months Ended		3 Months Ended		6 Months Ended				6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 2.0% Convertible Senior Notes, due May 15, 2012 [Member]	Jun. 30, 2011 2.0% Convertible Senior Notes, due May 15, 2012 [Member]	Jun. 30, 2012 2.0% Convertible Senior Notes, due May 15, 2012 [Member]	Jun. 30, 2011 2.0% Convertible Senior Notes, due May 15, 2012 [Member]	Jul. 30, 2012 2.0% Convertible Senior Notes, due May 15, 2012 [Member]	Dec. 31, 2011 2.0% Convertible Senior Notes, due May 15, 2012 [Member]	Jun. 30, 2012 7.5% Senior Notes, due November 15, 2019 [Member]	Jan. 18, 2012 7.5% Senior Notes, due November 15, 2019 [Member]	Dec. 31, 2011 7.5% Senior Notes, due November 15, 2019 [Member]	Nov. 16, 2011 7.5% Senior Notes, due November 15, 2019 [Member]
Long-Term Debt (Textual) [Abstract]
Long-term debt, principal outstanding										$ 475,000,000		$ 400,000,000
Debt instrument, interest rate, stated percentage			2.00%		2.00%			2.00%	7.50%		7.50%
Earliest date of expiration associated with warrants issued in conjuction with the 2.0% Convertible Senior Notes					Aug. 13, 2012
Latest date of expiration associated with warrants issued in conjunction with the 2.0% Convertible Senior Notes					Oct. 22, 2012
Premium percentage at which notholders can put notes back to the company upon a change in control									101.00%
fair value expressed as a percentage of the principal value									108.50%
Debt instrument maturity date			May 15, 2012						Nov. 15, 2019
Semi-annual interest payment due dates									May 15 and November 15
Interest payment commencement date									May 15, 2012
Principal amount of additional notes issued										75,000,000
Premium percentage on debt issuance										103.75%
Debt instrument, issuance date			March 28, 2007
Date of issuance of shares and cash in satisfaction of the conversion of outstanding notes			May 15, 2012
Number of shares issued in satisfaction of the conversion value in excess of the principal amount of notes converted			2,127,399
Common stock issued upon conversion of the 2.0% Convertible Senior Notes, value	129,431,000	0			129,400,000
Cash paid in satisfaction of the principal amount of notes converted			259,900,000
Interest paid at time of conversion			2,600,000
Convertible senior notes carrying amount								257,000,000
Unamortized discount of debt instruments								2,900,000
Initial conversion ratio into shares of common stock per note converted			23.5114
Conversion price per share			$ 42.53		$ 42.53
Average market price of the Company's common stock			$ 63.23	$ 66.37	$ 65.26	$ 60.54
Shares of common stock covered under convertible note hedge transaction			6,112,964		6,112,964
Shares of common stock covered under warrants			6,112,964		6,112,964
Strike price of warrants			53.77		53.77
If-settled value per dollar above the warrant exercise price			6,100,000		6,100,000
Closing price of Company's common stock			$ 65.91		$ 65.91		$ 90.10
Value of common stock owed to warrant holders if warrants are exercised			$ 74,200,000		$ 74,200,000		$ 222,100,000
Shares of common stock owed to warrant holders if warrants are exercised			1,126,000		1,126,000		2,465,000