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Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 30,575	$ 18,468	$ 88,049	$ 34,271
Income Statement Location [Axis]: us-gaap:ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	14,227	7,624	38,416	12,853
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	15,887	10,606	48,824	20,849
Income Statement Location [Axis]: mirm:CostOfSales
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	461	$ 238	$ 809	$ 569
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value (in dollars per share)			$ 58.44	$ 31.62
Stock-based compensation, Intrinsic value of options exercised			$ 65,200	$ 34,100
Total unrecognized stock-based compensation related to unvested stock option awards granted	$ 99,800		$ 99,800
Unrecognized stock-based compensation related to unvested stock, expected to recognize over weighted-average period			2 years 9 months 18 days
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation related to unvested stock option awards granted	$ 106,600		$ 106,600
Unrecognized stock-based compensation related to unvested stock, expected to recognize over weighted-average period			2 years 4 months 24 days
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			3 years
2023 Executive PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation related to unvested stock, expected to recognize over weighted-average period			1 year 9 months 18 days
Total unrecognized stock-based compensation	$ 20,500		$ 20,500
Bluejay Employee Stock Awards \| Income Statement Location [Axis]: us-gaap:ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			15,400
Bluejay Employee Stock Awards \| Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 19,300