Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Quarterly financial information
Loss from operations	$ (82,523)	$ (49,520)	$ (58,360)	$ (29,964)	$ (25,547)	$ (21,180)	$ (15,798)	$ (20,889)	$ (220,367)	$ (83,415)	$ (38,573)
Interest income	858	881	331	246					2,316	403	15
Net loss	$ (81,665)	$ (48,639)	$ (58,029)	$ (29,718)	$ (25,375)	$ (21,082)	$ (15,719)	$ (20,836)	$ (218,051)	$ (83,012)	$ (38,474)
Basic and diluted net loss per common share (in dollars per share)	$ (2.55)	$ (1.52)	$ (2.07)	$ (1.07)	$ (0.92)	$ (0.87)	$ (0.68)	$ (1.24)	$ (7.28)	$ (3.67)	$ (5.43)
Weighted-average basic and diluted common shares outstanding (in shares)	32,030	31,941	27,972	27,734	27,678	24,166	23,014	16,775	29,935	22,648	7,088