Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Income Tax Disclosure [Line Items]
Provision for income taxes		$ 0
Income tax position description		For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority.
Federal operating loss carryforwards		$ 235,400,000
State operating loss carryforwards		$ 234,300,000
Federal and state net operating loss carryforwards expiration year		2031
Federal orphan drug tax credit carry forwards		$ 29,800,000
Tax credit carry forwards expiration period		2034
Change in amount of deferred tax assets		$ 66,000,000
Valuation allowance		135,072,000	$ 69,069,000
Unrecognized tax benefit related to start-up costs		0		$ 2,880,000
Accrued interest or penalties related to uncertain tax positions		0	0
Uncertain tax positions		0
ASU 2016-09 [Member] | Scenario, Forecast [Member]
Income Tax Disclosure [Line Items]
Increase in deferred tax assets associated with net operating losses	$ 8,000,000
Increase in deferred tax assets associated with federal and state research credits	$ 1,500,000
ASU 2015-17, Balance Sheet Classification of Deferred Taxes [Member] | ASU Early Adoption [Member]
Income Tax Disclosure [Line Items]
Decrease in deferred tax liability current			(600,000)
Increase in deferred tax liability non-current			$ 600,000
Tax Deductions from Exercise of Stock Options [Member]
Income Tax Disclosure [Line Items]
Net operating losses exclude from deferred tax assets		8,000,000
Federal and state research and development credits exclude from deferred tax assets		1,500,000
Domestic Tax Authority [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards		$ 4,100,000
Research and development tax credits carryforwards expiration year		2031
Excess equity based compensation, tax deductions		$ 20,400,000
Excess equity based compensation, tax credits		1,300,000
State and Local Jurisdiction [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards		$ 1,600,000
Research and development tax credits carryforwards expiration year		2027
Excess equity based compensation, tax deductions		$ 20,400,000
Excess equity based compensation, tax credits		$ 200,000