Stock-Based Compensation - Summary of Stock Awards Granted (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended				12 Months Ended
	Jun. 30, 2024		Jun. 30, 2023	Jun. 30, 2024		Jun. 30, 2023		Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Stock options	5,100		54,575	321,478		1,084,924
Expanded Grants, Stock options	54,905	[1]	33,752	1,216,890	[1]	1,109,663	[2]
Stock-based compensation expense	$ 50.5		$ 47.4	$ 91.2		$ 88.6
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Stock options	8,860			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 three and six months ended June 30, 2024 are 8,860 and 97,460 shares with performance conditions (“March 2024 PSUs”), respectively, 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 three and six months ended June 30, 2024. As of June 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 six months ended June 30, 2024 have only service-based criteria and vest over four years.

[2] Included in the RSUs for the six months ended June 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.