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Long-Term Debt (Details) (USD $)
6 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Debt Instrument [Line Items]    
Long-term debt outstanding $ 200,000,000 $ 200,000,000
Senior Convertible Note [Member]
   
Debt Instrument [Line Items]    
Long-term debt outstanding 200,000,000  
Senior notes, maturity date Dec. 01, 2018  
Debt Instrument, Convertible, Earliest Date May 28, 2012  
Weighted Average Price Of Common Stock $ 111.54  
Senior notes, fair value 210,500,000 206,500,000
Interest rate 0.00%  
Senior Convertible Note [Member] | Minimum [Member]
   
Debt Instrument [Line Items]    
Trading Days Prior To Conversion Date Required For Conversion 50  
Senior Convertible Note [Member] | Maximum [Member]
   
Debt Instrument [Line Items]    
Trading Days Prior To Conversion Date Required For Conversion 65  
Eight Point Five Zero Percentage Senior Notes [Member]
   
Debt Instrument [Line Items]    
Long-term debt outstanding 200,000,000  
Senior notes, maturity date Nov. 15, 2017  
Senior notes, fair value $ 210,500,000 $ 206,500,000
Interest rate 8.50%