Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
Increase in valuation allowance	$ 4,800	$ 6,500
Unrecognized tax benefits	10,116	$ 4,061	$ 332	$ 287
Employee Stock Option [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	2,100
Federal [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	68,400
Federal [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits	$ 955
Tax credit, expiration	Begin to expire in 2029
Federal [Member] | Orphan Drug Credit [Member]
Income Tax Contingency [Line Items]
Tax credits	$ 20,900
Tax credit, expiration	Begins to expire in 2033
State [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 55,100
Operating loss carryforwards, expiration	Begin to expire in 2029
State [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits	$ 1,616
Foreign [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 17,400
Operating loss carryforwards, expiration	Begin to expire in 2023