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Convertible Senior Notes - Additional Information (Detail) (USD $)
 1 Months Ended 3 Months Ended 9 Months Ended
Jul. 24, 2013
Jun. 30, 2013
Apr. 01, 2013
Jan. 03, 2012
Nov. 30, 2009
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Debt Instrument [Line Items]                    
Debt instrument, maturity date               Apr. 02, 2013    
Gain on debt extinguishment               $ 84,000    
Debt discount         13,700,000          
4.50% Convertible senior notes (due 2014)
                    
Debt Instrument [Line Items]                    
Long term debt, face amount         100,000,000          
Debt instrument, interest rate         4.50%          
Frequency of interest payment         Semi-annually          
Debt instrument, maturity date         Nov. 01, 2014     Nov. 01, 2014    
Conversion rate in share per $1000 principal amount of notes         63.2091          
Debt instrument, conversion rate         $ 15.82          
Cash dividend on common stock per share paid   $ 0.07 $ 0.07 $ 0.10            
Net proceeds from debt instrument 95,900,000                  
Interest expenses associated with amortization of equity component           700,000 700,000 2,500,000 2,000,000  
Repurchase of related equity component               2,800,000    
Debt instrument unamortized discount           1,200,000   1,200,000   5,100,000
Debt discount writtent off amount               2,200,000    
Debt issuance cost written off amount           600,000   600,000    
Debt instrument repurchase amount 61,000,000                  
Long term debt, Fair Value           36,500,000   36,500,000   107,900,000
4.50% Convertible senior notes (due 2014) | Scenario, Forecast
                    
Debt Instrument [Line Items]                    
Conversion rate in share per $1000 principal amount of notes         68.8564          
Debt instrument, conversion rate         $ 14.52          
4.25% Convertible Senior Notes due 2018
                    
Debt Instrument [Line Items]                    
Long term debt, face amount 100,000,000                  
Debt instrument, interest rate 4.25%                  
Frequency of interest payment Semi-annually                  
Debt instrument, maturity date Aug. 01, 2018                  
Conversion rate in share per $1000 principal amount of notes 114.3674                  
Debt instrument, conversion rate $ 8.74                  
Net proceeds from debt instrument 100,000,000                  
Gain on debt extinguishment 100,000                  
Long term debt, Fair Value           $ 76,800,000   $ 76,800,000