Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Total stock based compensation expense
Subtotal	$ 17,026	$ 10,295
Capitalized stock-based compensation costs	(167)
Total	16,859	10,295
Research and development
Total stock based compensation expense
Total	7,798	5,790
General and administrative
Total stock based compensation expense
Total	9,061	4,505
Stock Options
Total stock based compensation expense
Subtotal	13,716	9,579
Restricted stock units
Total stock based compensation expense
Subtotal	3,153	624
Employee Stock
Total stock based compensation expense
Subtotal	$ 157	$ 92