Organization and Basis of Presentation - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Segment	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 21, 2016 USD ($)
Organization and Basis of Presentation [Line Items]
Accumulated deficit	$ (765,645,000)		$ (765,645,000)			$ (645,211,000)
Number of operating segment | Segment			1
Substantial doubt about going concern, conditions or events			The Company follows the provisions of the Financial Accounting Standards Board, or the FASB, Accounting Standard Codification, or ASC, Topic 205-40, Presentation of Financial Statements – Going Concern, which requires that management evaluate whether there are relevant conditions and events that in aggregate raise substantial doubt about the entity’s ability to continue as a going concern and to meet its obligations as they become due within one year from the date that the financial statements are issued.
Debt instrument, covenant description			In performing its evaluation, management first assessed the Company’s ability to satisfy a covenant under the indenture for its 0% Convertible Senior Secured Notes due 2020, or the 2016 Notes, that provides the noteholders with a redemption right after June 30, 2018 if consolidated net product sales (a non-GAAP measure defined in the indenture as the Company’s net sales plus aggregate net sales by the Company’s distributors outside the United States) for fiscal year 2017 are less than $100 million. Given the Company’s and its distribution partners’ limited history of selling and forecasting sales for Contrave and Mysimba in the United States and worldwide, it is difficult to accurately project whether consolidated net product sales for fiscal year 2017 will equal or exceed $100 million. The Company’s current forecast, however, projects that this requirement may not be met, such that the Company would be required to offer to redeem all of the 2016 Notes. Furthermore, the Company does not currently have adequate capital resources to redeem the entire $165 million in aggregate principal amount of the 2016 Notes if all of the notes were tendered. The Company is currently pursuing a number of potential actions to enable it to address its potential redemption obligations in the event that the covenant were not met. These actions include attempting to negotiate waivers or amendments with holders of the 2016 Notes, pursue recapitalization or restructuring transactions involving the 2016 Notes and/or the Company’s other indebtedness, which could also potentially result in raising additional working capital, and pursue various other strategic transactions that could result in repayment of the indebtedness, including a merger or sale of the Company. The Company, however, cannot guarantee that any of these potential actions would be successful. As a result, due to the uncertainty regarding the Company’s ability to comply with this covenant and redeem all of the 2016 Notes if they were tendered, and the uncertainty regarding the Company’s ability to conclude one or more of the actions it is pursuing to address the potential redemption requirements, management has concluded that there is substantial doubt regarding the Company’s ability to continue as a going concern and to meet all of its obligations as they become due within one year after the accompanying financial statements are issued.
Net product sales	$ 18,768,000	$ 5,241,000	$ 61,041,000	$ 8,176,000
Substantial doubt about going concern, management's plans, substantial doubt alleviated			The Company’s operating plan projects that the Company’s existing working capital can fund its business operations for one year after the accompanying financial statements are issued. This plan is based on management’s best estimate and assumptions for U.S. sales of Contrave, for which the Company reacquired the rights to sell in August 2016, and sales of Contrave or Mysimba™ outside the U.S. for which there is limited historical information. Due to the inherent uncertainty in achieving the forecasted global revenues in the plan, the Company has identified certain forecasted expenses that can be reduced starting in the first quarter of 2018 and through November 2018 if revenues are less than forecasted or if the Company is not able to raise additional equity or debt financing during the interim period.
Substantial doubt about going concern, management's plans, substantial doubt not alleviated			The identified cost-cutting actions could include a reduction of certain discretionary sales, general and administrative expenses. Management has evaluated that, if required, the cost cutting measures described above could be effectively implemented during the first quarter of 2018 and through November 2018, and that when implemented, would allow the Company to reduce its working capital requirements.
0% Convertible Senior Secured Notes due 2020 (2016 Notes)
Organization and Basis of Presentation [Line Items]
Interest rate	0.00%		0.00%				0.00%	0.00%
Debt instrument, principal amount	$ 1,000		$ 1,000				$ 165,000,000	$ 165,000,000
Scenario Forecast | 0% Convertible Senior Secured Notes due 2020 (2016 Notes) | Maximum
Organization and Basis of Presentation [Line Items]
Net product sales					$ 100,000,000