Net Loss Per Share - Potential Shares of Common Stock Excluded from Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Impairment Effects on Earnings Per Share [Line Items]
Options to purchase common shares	6,547,250	4,820,750	4,340,750
Outstanding warrants and options to purchase warrants	25,008,082	27,443,728	27,443,727
Potential shares of common stock excluded from diluted net loss per share	74,529,401	70,057,726	65,061,481
Amended and Restated Convertible Notes [Member]
Impairment Effects on Earnings Per Share [Line Items]
Amended and Restated notes	37,233,561	32,717,484	28,748,424
Amended and Restated Reimbursement Notes [Member]
Impairment Effects on Earnings Per Share [Line Items]
Amended and Restated notes	1,510,682	1,365,606	1,274,334
Bridge Notes [Member]
Impairment Effects on Earnings Per Share [Line Items]
Amended and Restated notes	4,229,826	3,710,158	3,254,246