Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Line Items]
Income tax expense (benefit), foreign	$ 0	$ 0	$ 0
Federal statutory income tax rate, percent	34.00%
Increase in valuation allowance	$ 28,000,000	9,500,000
Income tax effects of gains on available-for-sale securities recognized in other comprehensive income	435,000	0	$ 0
Research and development and orphan drug tax credit carryforwards	10,890,000
Reserve for uncertain tax positions placed against credit carryforwards	100,000
Reserve for uncertain tax positions placed against qualified expenses	$ 0	0
Cumulative changes in ownership Interest percentage threshold, subject to annual limitation of NOL and tax credit carryforwards	50.00%
Ownership interest change period	3 years
Beginning Tax Year [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards, expiration year	2035
Domestic Country
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 66,400,000	21,200,000
Research and development and orphan drug tax credit carryforwards	$ 11,000,000
Domestic Country | Beginning Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating Loss Carryforwards Expiration Year	2034
U. S State Tax Authority [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 55,000,000	$ 34,200,000
U. S State Tax Authority [Member] | Beginning Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating Loss Carryforwards Expiration Year	2035