Stock-Based Compensation - Restricted Stock Units - General Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 30,885	$ 15,656	$ 5,250
Service Based RSUs
Stock Options Granted to Non-Employees
Vesting period (in years)	3 years
Stock-based compensation expense	$ 700	800
Unrecognized compensation cost	$ 400
Weighted-average period expected to recognize stock-based compensation expense	8 months 12 days
Research and development expenses
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 15,383	5,988	1,294
General and administrative expenses
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 15,502	$ 9,668	$ 3,956