Income Taxes - Reconciliation of statutory rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective income tax rate, Amount
Federal statutory rate	$ (74,137)	$ (28,224)	$ (13,081)
Permanent differences	(1,203)	3,991	6,077
State taxes	(11,498)	(2,453)	(1,275)
Change in statutory rate	37,162
Research and development credit	(4,989)	(8,891)	(220)
Valuation allowance	56,049	35,502	8,472
Other	$ (1,384)	$ 75	$ 27
Tax Rate (as a percent)
Federal statutory rate (as a percent)	34.00%	34.00%	34.00%
Permanent differences (as a percent)	0.55%	(4.73%)	(15.80%)
State taxes (as a percent)	5.27%	2.97%	3.31%
Change in statutory rate	(17.04%)
Research and development credit (as a percent)	2.29%	10.55%	0.57%
Valuation allowance (as a percent)	(25.70%)	(42.70%)	(21.38%)
Other (as a percent)	0.63%	(0.09%)	(0.70%)