Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock-based compensation expense
Total stock-based compensation expense	$ 48,455	$ 25,914	$ 11,683
Research and development
Stock-based compensation expense
Total stock-based compensation expense	19,783	11,082	4,954
General and Administrative Expense.
Stock-based compensation expense
Total stock-based compensation expense	$ 28,672	$ 14,832	$ 6,729