Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Quarterly Financial Data [Abstract]
Total revenue	$ 13,498	$ 14,933	$ 11,049	$ 8,923	$ 16,195	$ 27,148	$ 18,507	$ 13,345	$ 48,403	$ 75,195	$ 96,023
Operating loss	(5,808)	(3,851)	(7,805)	(14,693)	(6,988)	(4,060)	(7,150)	(9,344)	(32,157)	(27,542)	(20,118)
Net loss	$ (5,995)	$ (4,248)	$ (7,281)	$ (15,252)	$ (6,925)	$ (2,768)	$ (7,325)	$ (10,355)	$ (32,776)	$ (27,373)	$ (23,139)
Net loss per share ― basic (in dollars per share)	$ (0.30)	$ (0.21)	$ (0.38)	$ (0.91)	$ (0.50)	$ (0.20)	$ (0.53)	$ (0.76)	$ (1.73)	$ (1.98)	$ (1.76)
Net loss per share ― diluted (in dollars per share)	$ (0.30)	$ (0.21)	$ (0.38)	$ (0.91)	$ (0.50)	$ (0.20)	$ (0.53)	$ (0.76)	$ (1.73)	$ (1.98)	$ (1.76)