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Debt - January 2017 Exchangeable Senior Notes - Additional Information (Detail)
 1 Months Ended 12 Months Ended
Jan. 20, 2017
USD ($)
d
$ / shares
Jan. 19, 2017
$ / shares
Oct. 31, 2018
USD ($)
shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Debt Disclosure [Line Items]            
Convertible senior notes, exchanged       $ 29,490,000   $ 128,115,000
Carrying amount of debt conversion option       29,369,000   127,436,000
Transaction costs       121,000   680,000
Interest expense, non-cash       2,183,000 $ 2,332,000 7,783,000
Common Shares            
Debt Disclosure [Line Items]            
Carrying amount of debt conversion option       5,011,000   $ 40,062,000
Convertible Debt | 2017 Notes            
Debt Disclosure [Line Items]            
Convertible senior notes, principal amount $ 30,000,000          
Convertible senior notes, exchanged     $ 30,000,000      
Convertible senior notes, stated interest rate 3.50%          
Convertible senior notes, due date 2047          
Convertible senior notes, repurchase price at option of the issuer 100.00%          
Net proceeds from offering $ 28,800,000          
Convertible senior notes, outstanding amount       $ 0    
Convertible senior notes redemption, end date Jan. 15, 2047          
Convertible senior notes, mandatory exchange in whole or in part if daily VWAP equaled or exceeded percentage of exchange price 130.00%          
Convertible senior notes, mandatory exchange in whole or in part if daily VWAP, stock trigger price | $ / shares $ 5.05          
Convertible senior notes, mandatory exchange in whole or in part if daily VWAP equaled or exceeded percentage of exchange price, at least number of VWAP Trading days | d 20          
Convertible senior notes, mandatory exchange in whole or in part if daily VWAP equaled or exceeded percentage of exchange price, in any number of VWAP Trading days | d 30          
Convertible senior notes, percentage of premium initial exchange price   35.00%        
Convertible senior notes, total ADS into which the debt is exchangeable | shares       0    
Convertible senior notes, carrying value, exchanged     29,200,000      
Equity component of convertible notes issuance cost     24,400,000      
Debt instrument increase accrued interest     300,000      
Convertible senior notes, interest payable dates       payable semi-annually in arrears on January 15 and July 15 of each year, beginning on July 15, 2017    
Convertible senior notes, payment start date Jul. 15, 2017          
Convertible senior notes, maturity date Jan. 15, 2047          
Convertible senior notes, discount $ 1,200,000     $ 0 1,000,000  
Convertible notes, carrying amount net of unamortized discount         29,000,000  
Convertible notes, interest expense       1,100,000 1,200,000  
Interest expense, contractual coupon interest       900,000 1,000,000  
Accrued interest on convertible note         500,000  
Interest payments on notes       1,100,000 500,000  
Convertible Debt | 2017 Notes | Non Cash            
Debt Disclosure [Line Items]            
Interest expense, non-cash       $ 200,000 $ 200,000  
Convertible Debt | 2017 Notes | Common Shares            
Debt Disclosure [Line Items]            
Carrying amount of debt conversion option     $ 5,000,000      
Convertible Debt | 2017 Notes | American Depositary Shares            
Debt Disclosure [Line Items]            
Convertible senior notes, exchange rate of ADS per $1,000 principal amount of notes 257.2016   257.2016      
Convertible senior notes, initial exchange price | $ / shares $ 3.89 $ 2.88        
Convertible senior notes, total ADS into which the debt is exchangeable | shares     7,716,046      
Transaction costs     $ 100,000