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Debt - January 2017 Exchangeable Senior Notes - Additional Information (Detail)
 9 Months Ended
Nov. 02, 2018
USD ($)
shares
Jan. 20, 2017
USD ($)
d
$ / shares
Jan. 19, 2017
$ / shares
Sep. 30, 2018
USD ($)
shares
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Debt Disclosure [Line Items]            
Interest expense, non-cash       $ 1,687,000 $ 1,736,000  
Convertible Debt | 2017 Notes            
Debt Disclosure [Line Items]            
Convertible senior notes, principal amount   $ 30,000,000   $ 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, total ADS into which the debt is exchangeable | shares       7,716,048    
Accrued interest on convertible note       $ 200,000   $ 200,000
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, percentage of premium initial exchange price     35.00%      
Value of shares in excess of principal amount of Notes based on closing price       $ 95,500,000    
Convertible senior notes, redemption start date   Jan. 19, 2021        
Convertible senior notes redemption, end date   Jan. 15, 2047        
Convertible senior notes, mandatory exchange in whole or in part if daily VWAP equals or exceeds 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 equals or exceeds 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 equals or exceeds percentage of exchange price, in any number of VWAP Trading days | d   30        
Convertible senior notes, discount   $ 1,200,000   800,000   1,000,000
Convertible notes, carrying amount net of unamortized discount       29,200,000   $ 29,000,000
Convertible notes, interest expense       1,000,000 900,000  
Interest expense, contractual coupon interest       800,000 700,000  
Interest payments on notes       1,100,000 500,000  
Convertible Debt | 2017 Notes | Minimum            
Debt Disclosure [Line Items]            
Percentage of holders of principal amount required to issue notice on default   25.00%        
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        
Convertible senior notes, initial exchange price | $ / shares   $ 3.89 $ 2.88      
Convertible Debt | 2017 Notes | Scenario Forecast            
Debt Disclosure [Line Items]            
Convertible senior notes, outstanding amount $ 0          
Accrued interest on convertible note $ 0          
Convertible Debt | 2017 Notes | Scenario Forecast | American Depositary Shares            
Debt Disclosure [Line Items]            
Convertible senior notes, total ADS into which the debt is exchangeable | shares 7,716,048          
Convertible Debt | 2017 Notes | Non Cash            
Debt Disclosure [Line Items]            
Interest expense, non-cash       $ 200,000 $ 100,000