Fair Value Measurements - Narrative (Details)		1 Months Ended				3 Months Ended		9 Months Ended
Fair Value Measurements - Narrative (Details)	Aug. 04, 2021 shares	Aug. 31, 2024 shares	Aug. 31, 2023 shares	Aug. 31, 2022 shares	Aug. 31, 2021 sale	Sep. 30, 2025 USD ($) shares	Sep. 30, 2021 shares	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchase of marketable securities \| $								$ (2,092,014,000)	$ (999,782,000)
Maturities of marketable securities \| $								1,654,000,000	$ 300,000,000
Marketable securities \| $						$ 1,345,222,000		1,345,222,000		$ 878,796,000
Current contingent consideration \| $						42,500,000		42,500,000		24,700,000
Contingent consideration \| $						$ 259,600,000		$ 259,600,000		144,200,000
Probabilities of success
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input						0.39		0.39
Motus
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued in acquisition (in shares) \| shares	2,889,367	182,182	177,203	171,427			3,420,149
Remaining deferred consideration \| $										$ 0
Shares issued upon milestone achievements (in shares) \| shares	5,348,572
Motus \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current contingent consideration \| $						$ 42,500,000		$ 42,500,000
Contingent consideration \| $						$ 259,600,000		$ 259,600,000
Motus \| Development Milestone
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued in acquisition (in shares) \| shares						364,566
Motus \| Development and regulatory milestones
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued upon milestone achievements (in shares) \| shares	4,979,705
Proceeds of sale of priority review voucher obligated to pay to equity holders, percentage					50.00%
Average sales prices of publicly disclosed priority review voucher sales obligated to pay equity holders, percentage					50.00%
Number of publicly disclosed sales used to determine average sales price owed to equity holders \| sale					3
Motus \| First Anniversary of Closing Date
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued in acquisition (in shares) \| shares	184,433
Motus \| Second Anniversary of Closing Date
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued in acquisition (in shares) \| shares	184,433
Motus \| Third Anniversary of Closing Date
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued in acquisition (in shares) \| shares	184,433
AlgaeneX
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued upon milestone achievements (in shares) \| shares	368,867

INSMED Inc 10-Q