Restructuring Costs - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 1,506	$ 50	$ 2,734	$ 203
Fixed asset impairment						$ 66,087
2019 French Restructuring Obligation
Restructuring Cost and Reserve [Line Items]
Restructuring costs			1,939
Charges for employee severance, benefits and other costs			2,414
Fixed asset impairment			525
Benefit related to the reversal of retirement indemnity obligation	1,000		1,000
Restructuring reserve	1,102		1,102			0
2019 French Restructuring Obligation | Scenario, Forecast
Restructuring Cost and Reserve [Line Items]
Severance costs					$ 3,500
2019 French Restructuring Obligation | Accrued Liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	1,078		1,078			24
2019 Corporate Restructuring Obligations
Restructuring Cost and Reserve [Line Items]
Restructuring costs	435		963
Charges for employee severance, benefits and other costs	541		2,359
Share-based compensation forfeitures related to the employees affected by the global reduction in workforce	976		1,396
Restructuring reserve	343		343			0
2019 Corporate Restructuring Obligations | Scenario, Forecast
Restructuring Cost and Reserve [Line Items]
Severance costs					$ 3,000
Lyon France Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring costs			168	203
Charges for employee severance, benefits and other costs			(168)	203
Restructuring reserve	$ 56	$ 633	$ 56	$ 633		$ 879	$ 1,000
Employee Severance | Corporate Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Expected number of positions eliminated (as a percent)	50.00%		50.00%
Employee Severance | Lyon France Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Expected number of positions eliminated (as a percent)								50.00%