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Long-Term Debt - Narrative (Details)
 3 Months Ended 9 Months Ended
Feb. 16, 2018
USD ($)
day
$ / shares
Rate
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
Mar. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Schedule of Capitalization, Long-term Debt [Line Items]                      
Interest expense, debt   $ 1,929,000 $ 3,259,000 $ 5,788,000 $ 9,686,000            
Coupon interest expense   1,617,000 1,617,000 4,851,000 4,851,000            
Amortization of debt discount and debt issuance costs   312,000 1,642,000 937,000 4,835,000            
Stockholders' equity   98,957,000 171,200,000 98,957,000 171,200,000 $ 118,880,000 $ 136,504,000 $ 162,266,000 $ 182,125,000 $ 32,067,000 $ (29,199,000)
ADSs, conversion ratio | Rate 9269.56%                    
Additional paid-in capital                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Stockholders' equity   546,565,000 $ 565,440,000 546,565,000 $ 565,440,000 $ 544,094,000 $ 542,093,000 566,916,000 $ 615,207,000 $ 497,249,000 $ 434,391,000
Cumulative Effect, Period of Adoption, Adjustment                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Book value of long-term debt               12,939,000      
Stockholders' equity               (12,939,000)      
Cumulative Effect, Period of Adoption, Adjustment | Additional paid-in capital                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Stockholders' equity               (26,699,000)      
Senior Notes                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Book value of long-term debt   $ 142,086,000   $ 142,086,000       $ 128,210,000      
4.50% Exchangeable Senior Notes Due 2023 | Convertible Debt                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Debt instrument, face amount $ 125,000                    
Debt instrument, interest rate, stated percentage 4.50%                    
Option to purchase aggregate principal amount, term 30 days                    
Option to increase aggregate principal amount $ 18,750,000                    
Payments of debt issuance costs 6,190,000                    
Proceeds from loans or conditional grants $ 137,560,000                    
ADS, option price per share (in dollars per share) | $ / shares $ 10.79                    
ADS premium percentage 20.00%                    
ADS purchased, price per share (in dollars per share) | $ / shares $ 8.99                    
4.50% Exchangeable Senior Notes Due 2023 | Senior Notes                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Debt instrument, interest rate, stated percentage   4.50%   4.50%       4.50%      
Debt Instrument, Redemption, Period One | 4.50% Exchangeable Senior Notes Due 2023 | Convertible Debt                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Number of business days 5 days                    
Consecutive business days 5 days                    
Percentage of product of the last reported sale price of the American Depositary Shares and the exchange rate on each such trading day 98.00%                    
Debt Instrument, Redemption, Period Two | 4.50% Exchangeable Senior Notes Due 2023 | Convertible Debt                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Scheduled threshold trading days 35 days                    
Threshold trading days | day 95                    
Debt Instrument, Redemption, Period Three | 4.50% Exchangeable Senior Notes Due 2023 | Convertible Debt                      
Schedule of Capitalization, Long-term Debt [Line Items]                      
Threshold trading days | day 20                    
Consecutive trading days | day 30                    
Threshold percentage of stock price trigger 130.00%