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Convertible Senior Notes - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Nov. 01, 2014
Jun. 30, 2014
Jul. 31, 2013
Nov. 30, 2009
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2013
Dec. 31, 2014
Debt Instrument [Line Items]                  
Payment for repurchase of common stock           $ 6,988 $ 24,000    
Repurchase of common stock under prepaid forward repurchase agreement, shares   3,112,840              
4.50% Convertible senior notes (due 2014)                  
Debt Instrument [Line Items]                  
Long term debt, face amount       $ 100,000          
Debt instrument, interest rate       4.50%          
Frequency of interest payment           Semi-annually      
Debt instrument, maturity date       Nov. 01, 2014          
Repayments of debt $ 39,000                
Debt discount       $ 13,700          
Interest expenses associated with amortization of equity component         $ 300   $ 800    
Debt instrument repurchase amount     $ 61,000            
4.25% Convertible Senior Notes (due 2018)                  
Debt Instrument [Line Items]                  
Long term debt, face amount     $ 100,000            
Debt instrument, interest rate     4.25%            
Frequency of interest payment           Semi-annually      
Debt instrument, maturity date     Aug. 01, 2018            
Gain on debt extinguishment     $ 100            
Conversion rate in share per $1000 principal amount of notes     114.3674            
Debt instrument, conversion rate     $ 8.74            
Net proceeds from debt instrument     $ 96,000            
Long term debt, fair value           $ 106,900     $ 96,300
4.875% Convertible Senior Notes (due 2020)                  
Debt Instrument [Line Items]                  
Long term debt, face amount   $ 115,000              
Debt instrument, interest rate   4.875%              
Frequency of interest payment           Semi-annually      
Debt instrument, maturity date   Jun. 01, 2020              
Conversion rate in share per $1000 principal amount of notes   103.7613              
Debt instrument, conversion rate   $ 9.64              
Net proceeds from debt instrument   $ 110,400              
Payment for repurchase of common stock   $ 24,000              
Repurchase of common stock under prepaid forward repurchase agreement, shares   3,100,000              
Repayments of debt               $ 39,000  
Long term debt, fair value           $ 118,200     $ 100,900