Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
	Nov. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2021 USD ($) ft² shares	Jun. 30, 2021 USD ($) ft² shares	Dec. 31, 2020 USD ($) shares	Oct. 31, 2020 shares
Significant Accounting Policies [Line Items]
Restricted cash deposit		$ 2,160	$ 2,160	$ 327
Restricted cash noncurrent		2,070	2,070	327
Restricted cash, current		90	90	0
Operating lease, right-of-use assets		$ 1,581	$ 1,581	$ 3,579
Common stock, shares issued | shares		110,295,992	110,295,992	84,893,994
Payment of exchange fees	$ 313
Common stock shares issued warrant to purchase | shares	30,000
Common stock warrants exercise price per share | $ / shares	$ 3.29
Agreement termination period	60 days
Agreement written notice period	30 days
ASU 2018-13 [Member]
Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020	Jan. 01, 2020
Change in accounting principle, accounting standards update, adopted		true	true
Change in accounting principle, accounting standards update, immaterial effect		true	true
ASU 2019-12 [Member]
Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2021	Jan. 01, 2021
Change in accounting principle, accounting standards update, adopted		true	true
Change in accounting principle, accounting standards update, immaterial effect		true	true
Ultragenyx [Member] | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement [Member]
Significant Accounting Policies [Line Items]
Common stock, shares issued | shares					7,825,797
Office laboratory [Member] | Hood Park LLC [Member]
Significant Accounting Policies [Line Items]
Area of lease agreement | ft²		49,869	49,869
Lease description			The Company intends to relocate its corporate headquarters to the Premises in early 2022. The term of the lease commences on the later of (i) the date the Landlord delivers the Premises to the Company or (ii) the earlier of (a) the date the Company’s work on the Premises is substantially completed, (b) the date the Company commences business operations in the Premises, or (c) the one hundred twentieth (120th) day following the Landlord’s satisfaction of item (i) above. The lease commencement date is anticipated to be in the first quarter of 2022. The date on which the Company will become responsible for paying rent under the lease will be 61 days following the lease commencement date.
Lease term		10 years	10 years
Lease renewal term		5 years	5 years
Letter of credit amount		$ 1,833	$ 1,833
Tenant improvement allowance receivable		348	348
Lease liability		1,314	1,314
Operating lease, right-of-use assets		$ 1,233	$ 1,233