Organization and Operations - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Aug. 05, 2016	Apr. 30, 2016
Schedule Of Description Of Business [Line Items]
Accumulated deficit	$ (249,547,000)	$ (238,877,000)
Aggregate cash and cash equivalents and marketable securities	$ 114,705,000
2021 Convertible Notes [Member] | Senior Notes [Member]
Schedule Of Description Of Business [Line Items]
Aggregate principal amount			$ 50,000,000
Convertible senior notes, interest rate, stated percentage	5.75%
Cowen and Company, LLC [Member]
Schedule Of Description Of Business [Line Items]
Number of shares of common stock sold	0
Available for sale of common stock	$ 45,599,000
Amount of stock transactions, available balance under base prospectus	$ 98,000,000
Cowen and Company, LLC [Member] | 2021 Convertible Notes [Member] | Senior Notes [Member]
Schedule Of Description Of Business [Line Items]
Aggregate principal amount		$ 52,000,000
Convertible senior notes, interest rate, stated percentage		5.75%
Maximum [Member] | Cowen and Company, LLC [Member]
Schedule Of Description Of Business [Line Items]
Amount of stock transactions				$ 200,000,000
Amount of common stock transactions				$ 50,000,000