Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	$ 243,033	$ 187,221
Unrealized Gains	224	245
Unrealized Losses	(16)	(126)
Fair Value	243,241	187,340
U.S. Government and Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	123,444	44,281
Unrealized Gains	66	11
Unrealized Losses	(1)	(77)
Fair Value	123,509	44,215
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost		1,466
Unrealized Losses		(4)
Fair Value		1,462
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	119,589	141,474
Unrealized Gains	158	234
Unrealized Losses	(15)	(45)
Fair Value	$ 119,732	$ 141,663

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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+ Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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+ Details

REGENXBIO Inc. 10-Q