Basis of Presentation and Summary of Significant Accounting Policies - Accounting for Share-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basis of Presentation [Line Items]
Share-based compensation expense	$ 6,297	$ 5,470	$ 18,215	$ 14,837
Research and development expenses
Basis of Presentation [Line Items]
Share-based compensation expense	2,481	2,257	7,136	6,120
General and administrative expenses
Basis of Presentation [Line Items]
Share-based compensation expense	$ 3,816	$ 3,213	$ 11,079	$ 8,717