Stock-Based Compensation - Summary of Stock Awards Granted (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended				12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024		Sep. 30, 2023		Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Stock options	6,442	50,875	327,920		1,135,799
Expanded Grants, Stock options	135,092	27,539	1,351,982	[1]	1,137,202	[2]
Stock-based compensation expense	$ 43.5	$ 48.1	$ 134.6		$ 136.7
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Stock options			97,460		502,225		38,500
Remaining shares associated with performance condition			53,160
Vesting PSUs	44,300		97,460

[1] Included in restricted stock units (“RSUs”) for the nine months ended September 30, 2024 are 97,460 shares with performance conditions (“March 2024 PSUs”) which are related to the achievement of certain financial performance goals and regulatory approval of certain of the Company's product candidates. The expanded regulatory approval of ELEVIDYS in June 2024 resulted in the cliff-vesting of 44,300 of the 97,460 shares of the March 2024 PSUs during the nine months ended September 30, 2024. As of September 30, 2024, none of the performance conditions for the remaining 53,160 shares were probable of being achieved. Stock options and the remaining RSUs granted during the three and nine months ended September 30, 2024 have only service-based criteria and vest over four years.

[2] Included in the RSUs for the nine months ended September 30, 2023 are 502,225 shares with performance conditions (the “March 2023 PSUs”), which are related to regulatory approval of ELEVIDYS and achievement of a certain financial performance target, both of which were achieved in 2023. Vesting of the March 2023 PSUs is contingent on the fulfillment of remaining service conditions.