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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Stock-Based Compensation - Compensation expense (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Stock-based compensation expense
Stock-based compensation expense		$ 10,358	$ 9,887	$ 30,440	$ 18,402
Unrecognized compensation cost	$ 76,900	76,900		76,900
Unrecognized compensation cost expected to be recognized over a weighted average period	2 years 3 months 18 days
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense		2,195	1,347	6,798	2,331
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense		$ 8,163	$ 8,540	$ 23,642	$ 16,071