Long-Term Debt - Narrative (Details)		3 Months Ended		6 Months Ended
	Feb. 16, 2018 USD ($) day $ / shares Rate	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Capitalization, Long-term Debt [Line Items]
Interest expense, debt		$ 1,930,000	$ 3,237,000	$ 3,859,000	$ 6,427,000
Coupon interest expense		1,617,000	1,617,000	3,234,000	3,234,000
Amortization of debt discount and debt issuance costs		313,000	1,620,000	625,000	3,193,000
Stockholders' equity		118,880,000	182,125,000	118,880,000	182,125,000	$ 136,504,000	$ 162,266,000	$ 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		544,094,000	615,207,000	544,094,000	615,207,000	542,093,000	566,916,000	497,249,000	434,391,000
Accumulated deficit
Schedule of Capitalization, Long-term Debt [Line Items]
Stockholders' equity		(403,453,000)	$ (361,206,000)	(403,453,000)	$ (361,206,000)	$ (383,872,000)	(384,187,000)	$ (392,080,000)	$ (391,215,000)
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)
Cumulative Effect, Period of Adoption, Adjustment | Accumulated deficit
Schedule of Capitalization, Long-term Debt [Line Items]
Stockholders' equity							13,760,000
Senior Notes
Schedule of Capitalization, Long-term Debt [Line Items]
Book value of long-term debt		$ 141,774,000		$ 141,774,000			$ 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%