LEASES - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	May 31, 2023 USD ($)	Mar. 31, 2024 USD ($) ft² extension_option	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease, remaining lease term		2 years 5 months 12 days
Operating lease, cost		$ 1.2	$ 1.8
Operating lease, payments		$ 1.4	1.7
Loss on termination of lease				$ 0.5
Minimum
Lessee, Lease, Description [Line Items]
Operating lease, incremental borrowing rates based on remaining lease term		6.65%
Maximum
Lessee, Lease, Description [Line Items]
Operating lease, incremental borrowing rates based on remaining lease term		6.94%
Cambridge
Lessee, Lease, Description [Line Items]
Area of property leased (in square feet) | ft²		65,167
Cambridge | Other Assets | Letter of Credit
Lessee, Lease, Description [Line Items]
Total security deposit in connection with lease		$ 1.7		$ 1.7
Cambridge | Laboratory Space
Lessee, Lease, Description [Line Items]
Leased area subject to expiration | ft²		5,951
Operating lease extensions (in extension options) | extension_option		1
Cambridge | Office And Storage Space
Lessee, Lease, Description [Line Items]
Leased area subject to expiration | ft²		59,216
Operating lease extensions (in extension options) | extension_option		1
Lease extension period		5 years
Boston | Office Space
Lessee, Lease, Description [Line Items]
Area of property leased (in square feet) | ft²		27,924
Payments to transfer rights and obligations of operating lease	$ 1.3
Boston | Office Space | Keryx Biopharmaceuticals, Inc.
Lessee, Lease, Description [Line Items]
Sublease rental income			$ 0.3