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Debt - January 2017 Exchangeable Senior Notes - Additional Information (Detail)
 9 Months Ended
Jan. 20, 2017
USD ($)
d
$ / shares
Jan. 19, 2017
$ / shares
Sep. 30, 2017
USD ($)
shares
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Debt Disclosure [Line Items]          
Interest     $ 12,536,000 $ 13,839,000  
Convertible Debt          
Debt Disclosure [Line Items]          
Convertible senior notes, stated interest rate     3.50%    
Convertible notes, interest expense       6,900,000  
Accrued interest on convertible note     $ 0   $ 200,000
Interest     300,000 2,500,000  
Convertible Debt | Non Cash          
Debt Disclosure [Line Items]          
Convertible notes, interest expense       $ 4,000,000  
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, 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%      
Convertible senior notes, total ADS into which the debt is exchangeable | shares     7,716,048    
Value of shares in excess of principal value of Notes based on closing price     $ 0    
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   1,100,000    
Convertible notes, carrying amount net of unamortized discount     28,900,000    
Convertible notes, interest expense     900,000    
Interest expense, contractual coupon interest     700,000    
Accrued interest on convertible note     200,000    
Interest     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 | Maximum | Non Cash          
Debt Disclosure [Line Items]          
Convertible notes, interest expense     $ 100,000    
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        
Convertible senior notes, initial exchange price | $ / shares $ 3.89 $ 2.88