Stock-based Compensation - Stock-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
Total stock based compensation expense
Subtotal	$ 19,675	$ 13,666	$ 36,701	$ 23,961
Capitalized stock-based compensation costs	(147)		(314)
Total	19,528	13,666	36,387	23,961
Research and development
Total stock based compensation expense
Total	8,709	7,503	16,507	13,293
General and administrative
Total stock based compensation expense
Total	10,819	6,163	19,880	10,668
Stock Options
Total stock based compensation expense
Subtotal	14,629	12,016	28,344	21,595
Restricted stock units
Total stock based compensation expense
Subtotal	4,844	1,544	7,998	2,168
Employee Stock
Total stock based compensation expense
Subtotal	$ 202	$ 106	$ 359	$ 198

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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