Income Taxes - Deferred Tax Assets (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Net loss	$ (20,628)	$ (73,319)	$ (30,401)	$ (24,528)	$ (27,194)	$ (17,691)	$ (15,916)	$ (11,597)	$ (148,876)	$ (72,398)	$ (35,876)
Components of deferred tax assets
Net operating losses	48,220				44,927				48,220	44,927	20,086
Accrued expenses	644				687				644	687	53
Research and development expenses	9,639				2,103				9,639	2,103	2,344
Research and development credits	20,665				3,929				20,665	3,929	1,899
Stock options	7,178				5,186				7,178	5,186	3,065
Other temporary differences	155				12				155	12	2
Gross deferred tax assets	86,501				56,844				86,501	56,844	27,449
Deferred tax asset valuation allowance	$ (86,501)				$ (56,844)				$ (86,501)	$ (56,844)	$ (27,449)