Share-Based Compensation - Share-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	$ (6,804)	$ (11,362)	$ (14,935)	$ (24,884)
Total equity awards compensation expense	(7,159)	(11,713)	(15,662)	(25,595)
R&D
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(2,068)	(3,328)	(4,438)	(7,353)
Total equity awards compensation expense	(2,068)	(3,328)	(4,438)	(7,353)
S&O
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(1,572)	(3,892)	(5,190)	(10,093)
Total equity awards compensation expense	(1,572)	(3,892)	(5,190)	(10,093)
G&A
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(3,164)	(4,142)	(5,307)	(7,438)
Total equity awards compensation expense	(3,519)	(4,493)	(6,034)	(8,149)
RSUs
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(6,169)	(12,117)	(14,146)	(24,434)
RSUs | R&D
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(2,068)	(3,952)	(4,438)	(7,798)
RSUs | S&O
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(1,516)	(4,669)	(5,135)	(10,624)
RSUs | G&A
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(2,585)	(3,496)	(4,573)	(6,012)
Share options / BSPCE
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(635)	755	(789)	(450)
Share options / BSPCE | R&D
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	0	624		445
Share options / BSPCE | S&O
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(56)	777	(55)	531
Share options / BSPCE | G&A
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based compensation expense	(579)	(646)	(734)	(1,426)
BSAs
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Issuance of Stock and Warrants for Services or Claims	(355)	(351)	(727)	(711)
BSAs | R&D
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Issuance of Stock and Warrants for Services or Claims	0	0	0	0
BSAs | S&O
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Issuance of Stock and Warrants for Services or Claims	0	0	0	0
BSAs | G&A
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Issuance of Stock and Warrants for Services or Claims	$ (355)	$ (351)	$ (727)	$ (711)