Stock-Based Compensation - Compensation expense (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 30, 2017
Stock-based compensation expense
Stock-based compensation expense		$ 1,000	$ 41,271	$ 27,528	$ 17,903
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense			9,011	3,787	1,756
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense			32,260	23,741	$ 16,147
Employees, directors and non-employees
Stock-based compensation expense
Unrecognized compensation cost			$ 70,600
Unrecognized compensation cost expected to be recognized over a weighted average period			2 years 2 months 12 days
Chairman
Stock-based compensation expense
Issuance of common stock for one‑time bonus payment (in shares)		213,439
Stock options.
Stock-based compensation expense
Stock-based compensation expense				$ 2,200
Purchase of an aggregate			3,059,004
Shares issued			0
Shares cancelled			257,854
Stock options. | Performance-based vesting
Stock-based compensation expense
Purchase of an aggregate				447,000
Restricted Common Stock
Stock-based compensation expense
Stock-based compensation expense			$ 7,600	$ 7,300
Shares issued			99,314	1,559,401
Shares cancelled			201,250
Restricted Common Stock | Executive officer
Stock-based compensation expense
Shares issued						460,000
Market-Based Stock Options | Executive officer | Vesting based on service period
Stock-based compensation expense
Stock-based compensation expense			$ 900	$ 200
Purchase of an aggregate	164,400
Market-Based Stock Options | Executive officer | Performance-based vesting
Stock-based compensation expense
Stock-based compensation expense			$ 0
Purchase of an aggregate	193,400