Subsequent Events - Additional Information (Detail)	Jul. 31, 2019	Jan. 30, 2019 USD ($)	Jan. 29, 2019 USD ($) shares	Jan. 29, 2019 GBP (£) shares	Feb. 27, 2018 USD ($) shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares
Subsequent Event [Line Items]
Common stock, shares issued | shares						27,386,632	8,826,190
Private Placement [Member]
Subsequent Event [Line Items]
Common stock, shares issued | shares					5,797,102
Proceeds from issuance of private placement | $					$ 80,000,000
Offering costs | $					$ 2,600,000
JJDC [Member] | Private Placement [Member]
Subsequent Event [Line Items]
Common stock, shares issued | shares					2,898,550
Subsequent Event [Member] | Private Placement [Member]
Subsequent Event [Line Items]
Registration Payment Arrangement Liquidated Damages Percent	1.00%
Subsequent Event [Member] | License Agreement [Member]
Subsequent Event [Line Items]
License agreement amendment description			On January 29, 2019, the Company amended and restated the following agreements: (i) the License Agreement, dated February 4, 2015, as amended, between the Company and UCL Business, Plc (“UCLB”); (ii) the License Agreement, dated July 28, 2017, as amended, between the Company and UCLB; and (iii) the License Agreement, dated March 15, 2018, between the Company and UCLB to establish new stand-alone license agreements for the following inherited retinal disease programs: (a) achromatopsia (“ACHM”) caused by mutations in CNGB3; (b) ACHM caused by mutations in CNGA3; (c) X-linked retinitis pigmentosa (“XLRP”); and (d) RPE65-mediated IRD.	On January 29, 2019, the Company amended and restated the following agreements: (i) the License Agreement, dated February 4, 2015, as amended, between the Company and UCL Business, Plc (“UCLB”); (ii) the License Agreement, dated July 28, 2017, as amended, between the Company and UCLB; and (iii) the License Agreement, dated March 15, 2018, between the Company and UCLB to establish new stand-alone license agreements for the following inherited retinal disease programs: (a) achromatopsia (“ACHM”) caused by mutations in CNGB3; (b) ACHM caused by mutations in CNGA3; (c) X-linked retinitis pigmentosa (“XLRP”); and (d) RPE65-mediated IRD.
License arrangement upfront payment			$ 1,976,000	£ 1,500,000
Common stock, shares issued | shares			1,500,000	1,500,000
Subsequent Event [Member] | Collaboration Agreement [Member] | Janssen Pharmaceuticals Inc [Member]
Subsequent Event [Line Items]
Upfront payment | $		$ 100,000,000
Commercial milestone payments | $		$ 340,000,000
Subsequent Event [Member] | Clinical Collaborative Arrangement [Member] | Janssen Pharmaceuticals Inc [Member]
Subsequent Event [Line Items]
Percentage of royalties payable on annual net sales of licensed product statement		20.00%
Percentage of costs payable on annual net sales of licensed product statement		100.00%