Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended			85 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2012
Income Tax Contingency [Line Items]
Provision for federal income taxes	$ 0	$ 0	$ 0	$ 0
Increase in valuation allowance	6,500	12,700
Unrecognized tax benefits	4,061	$ 332	$ 287	4,061	$ 73
Employee Stock Option [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	1,800			1,800
Federal [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	91,400			91,400
Federal [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits	$ 864			864
Tax credit, expiration	Begin to expire in 2029
Federal [Member] | Orphan Drug Credit [Member]
Income Tax Contingency [Line Items]
Tax credits	$ 3,900			3,900
Tax credit, expiration	Begins to expire in 2033
State [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 95,400			95,400
Operating loss carryforwards, expiration	Begin to expire in 2029
State [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits	$ 1,016			$ 1,016