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Debt (Details 2) (USD $)
 0 Months Ended 3 Months Ended 12 Months Ended
Apr. 16, 2007
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2011
Debt        
Loss on extinguishment of debt       $ 3,349,000
Components of interest expense recorded on the notes        
Accretion of the discount on the Notes       1,260,000
Convertible Notes
        
Debt        
Original conversion rate, number of shares to be issued per $1000 of principal amount of notes (in shares) 36.7277      
Convertible debt principal amount, basis for exchange 1,000      
Conversion price of convertible notes (in dollars per share) $ 27.23 $ 50.59 $ 45.95  
Premium over the closing market price of common stock on April 16, 2007 (as a percent) 38.00%      
Principal of notes tendered for conversion     7,500,000  
Notes tendered, cash paid   98,100,000 7,500,000  
Notes tendered, shares issued upon conversion   1,660,095 111,318  
Loss on extinguishment of debt   3,000,000 300,000  
Components of interest expense recorded on the notes        
Contractual interest       2,025,000
Accretion of the discount on the Notes       1,260,000
Total interest expense on the Notes       $ 3,285,000
Effective interest rate (as a percent)       6.70%