Stock-Based Compensation - Summary of Stock-based Compensation Reflected in Unaudited Condensed Consolidated Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 30,575	$ 18,468	$ 88,049	$ 34,271
Income Statement Location [Axis]: us-gaap:ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	14,227	7,624	38,416	12,853
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	15,887	10,606	48,824	20,849
Income Statement Location [Axis]: mirm:CostOfSales
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 461	$ 238	$ 809	$ 569