Subsequent Events - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended	6 Months Ended
	Aug. 03, 2017	Jul. 31, 2017	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016
Subsequent Event [Line Items]
Settlement charge related to estimated royalties			$ 2,800,000	$ 2,800,000
Repayment of outstanding loan payable				$ 5,054,000	$ 2,551,000
Net proceeds from issue of common stock					$ 37,500,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Upfront fee to be paid on execution of agreements		$ 35,000,000
Net proceeds for debt issuance of offerings	$ 34,800,000
Repayment of outstanding loan payable	9,200,000
Debt instrument fee	$ 1,100,000
Common stock average price per share		$ 42.50
Net proceeds from issue of common stock		$ 354,000
Underwriting discounts, commission and offering expenses		$ 19,900
Subsequent Event [Member] | Underwritten Public Offering [Member]
Subsequent Event [Line Items]
Common stock issued		8.8
Subsequent Event [Member] | Underwritten Offering [Member]
Subsequent Event [Line Items]
Common stock issued		1.2
Subsequent Event [Member] | MidCap Financial Amended and Restated Credit and Security Agreement [Member]
Subsequent Event [Line Items]
Credit agreement interest rate per annum		6.25%
Variable interest rate description		one-month London Interbank Offered Rate
Origination fee, percentage		0.50%
Final payment fee, percentage		2.00%
Monthly principal payments commencement date		Jul. 01, 2018
Credit facility maturity date		2021-07
Subsequent Event [Member] | MidCap Financial Amended and Restated Credit and Security Agreement [Member] | Term Loan
Subsequent Event [Line Items]
Current borrowing capacity		$ 60,000,000
Subsequent Event [Member] | Midcap Financial Revolving Credit and Security Agreement [Member]
Subsequent Event [Line Items]
Credit agreement interest rate per annum		3.95%
Variable interest rate description		one-month LIBOR
Origination fee, percentage		0.50%
Credit facility maturity date		2021-07
Subsequent Event [Member] | Midcap Financial Revolving Credit and Security Agreement [Member] | Revolving Loan [Member]
Subsequent Event [Line Items]
Current borrowing capacity		$ 40,000,000
Line of credit facility additional tranche that may be increased		$ 20,000,000
Subsequent Event [Member] | BioMarin Pharmaceuticals, Inc. [Member] | U.S. [Member]
Subsequent Event [Line Items]
Royalty payment expiry period		2023-12
Subsequent Event [Member] | BioMarin Pharmaceuticals, Inc. [Member] | EU [Member]
Subsequent Event [Line Items]
Royalty payment expiry period		2024-09
Subsequent Event [Member] | Settlement Agreement [Member] | BioMarin Pharmaceuticals, Inc. [Member]
Subsequent Event [Line Items]
Upfront fee to be paid on execution of agreements		$ 20,000,000
Subsequent Event [Member] | License Agreement [Member] | BioMarin and Academisch Ziekenhuis Leiden [Member]
Subsequent Event [Line Items]
Upfront fee to be paid on execution of agreements		15,000,000
Subsequent Event [Member] | Maximum [Member]
Subsequent Event [Line Items]
Regulatory and sales milestone payments		$ 65,000,000
Percentage of royalty payments		8.00%
Subsequent Event [Member] | Minimum [Member]
Subsequent Event [Line Items]
Percentage of royalty payments		4.00%