IMPAIRMENT OF GOODWILL, INDEFINITE-LIVED INTANGIBLE ASSETS AND OTHER LONG-LIVED ASSETS	12 Months Ended
	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
IMPAIRMENT OF GOODWILL, INDEFINITE-LIVED INTANGIBLE ASSETS AND OTHER LONG-LIVED ASSETS	IMPAIRMENT OF GOODWILL, INDEFINITE-LIVED INTANGIBLE ASSETS AND OTHER LONG-LIVED ASSETS Three months ended March 31, 2023 During the three months ended March 31, 2023, as a result of the sustained decline in the Company’s stock price and related market capitalization, termination of the collaboration agreements with Biogen and Novartis, the Company performed an impairment assessment of goodwill, indefinite-lived intangible assets, and long-lived assets. The Company operates as a single reporting unit based on its business and reporting structure. For goodwill, a quantitative impairment assessment was performed using a market approach, whereby the Company’s fair value of equity was compared to its carrying value. The fair value of equity was derived using both the market capitalization of the Company and an estimate of a reasonable range of values of a control premium applied to the Company’s implied business enterprise value. The control premium was estimated based upon control premiums observed in comparable market transactions. This represents a level 2 nonrecurring fair value measurement. Based on this analysis, the Company recognized a pre-tax goodwill impairment charge of $38.1 million during the three months ended March 31, 2023. As a result, the goodwill was fully impaired as of March 31, 2023. Before completing the goodwill impairment assessment, the Company also tested its indefinite-lived intangible assets and then its long-lived assets for impairment. Based on the qualitative assessment, the Company determined it was more likely than not that its indefinite-lived intangible assets were not impaired. The Company determined all of its long-lived assets represent one asset group for purposes of long-lived asset impairment assessment. The Company concluded that the carrying value of the asset group was not recoverable as it exceeded the future undiscounted cash flows the assets are expected to generate from the use and eventual disposition. To allocate and recognize the impairment loss, the Company determined individual fair values of its long-lived assets. The Company applied a discounted cash flow method to estimate fair values of its leasehold improvements and right-of-use assets, including leasehold improvements in the process of construction and a cost replacement method to estimate the fair value of its furniture, fixtures and laboratory and manufacturing equipment. These represented level 3 nonrecurring fair value measurements. Based on this analysis, the Company recognized pre-tax long-lived asset impairment charges of $11.2 million on the right-of-use assets, $5.0 million on the related leasehold improvements, and $4.2 million on construction-in-progress, during the three months ended March 31, 2023. No impairment was recognized on the remaining long-lived assets as their carrying values were not in excess of their fair values. Three months ended June 30, 2023 During the three months ended June 30, 2023, the Company’s stock price and the related market capitalization continued to decline. In April 2023, the Company announced a restructuring of operations and a corresponding reduction in force, see Note 11 – Restructuring Charges. The Company also initiated discussions around several actions aimed at reducing costs, preserving liquidity and improving operational performance metrics. These actions include but are not limited to deferral and reprioritization of certain research and development programs, further reduction in force, and closing or downsizing its facilities. The Company reassessed its indefinite-lived and long-lived assets for impairment as of June 30, 2023. Given the actions contemplated above, the Company determined that it was more likely than not that its indefinite-lived intangible assets were impaired. Accordingly, the Company developed an estimate of the fair value of its indefinite-lived intangible assets using the multi-period excess earnings model (income approach) and concluded the carrying value of its indefinite-lived intangible assets were fully impaired. This represents a level 3 nonrecurring fair value measurement. As a result, an indefinite-lived intangible assets impairment charge of $51.4 million, as well as the related income tax benefit of $6.3 million due to the reversal of a deferred tax liability associated with the indefinite-lived intangible assets, were recognized during the three months ended June 30, 2023. The impairment charge was primarily driven by a higher discount rate applied to future cash flows based on market participants’ view of increased risk related to the asset. The Company determined that there were indicators of impairment in its long-lived asset group as of June 30, 2023, based on the same factors above as well as the impairment of its indefinite-lived intangible assets. As the estimated fair value of this asset group, based on a market approach, exceeded its carrying value, no impairment loss was recognized. This represented a level 3 nonrecurring fair value measurement. Three months ended September 30, 2023 During the three months ended September 30, 2023, the Company’s stock price and the related market capitalization continued to decline, and as such, the Company reassessed its long-lived assets for impairment as of September 30, 2023. The Company determined all of its long-lived assets continued to represent one asset group for purposes of long-lived asset impairment assessment. The Company concluded that the carrying value of the asset group was not recoverable and the estimated fair value of this asset group was below its carrying value. The lower fair value of the asset group was mainly driven by the sustained decline in the Company’s stock price and the related market capitalization. To recognize the impairment loss, the Company determined individual fair values of its long-lived assets. The Company applied a discounted cash flow method to estimate fair values of its leasehold improvements and right-of-use assets, including leasehold improvements in the process of construction, and a market approach to estimate the fair value of its furniture, fixtures and laboratory and manufacturing equipment. These represented level 3 nonrecurring fair value measurements. Based on this analysis, the Company concluded the fair values of the long-lived assets were lower than their net book values due to declines in the market prices for leases, furniture, fixtures, and equipment. The Company recognized pre-tax long-lived asset impairment charges of $17.6 million on the right-of-use assets, $13.7 million on the related leasehold improvements and construction-in-progress, and $13.5 million on furniture, fixtures, and laboratory and manufacturing equipment during the three months ended September 30, 2023. Three months ended December 31, 2023 During the three months ended December 31, 2023, the Company’s stock price and the related market capitalization continued to decline, and as such, the Company reassessed its long-lived assets for impairment as of December 31, 2023. The Company determined all of its long-lived assets continued to represent one asset group for purposes of long-lived asset impairment assessment. The Company concluded that the carrying value of the asset group was not recoverable and the estimated fair value of this asset group was below its carrying value. The lower fair value of the asset group was mainly driven by the decline in the Company’s stock price and the related market capitalization. To recognize the impairment loss, the Company determined individual fair values of its long-lived assets. The Company applied a discounted cash flow method to estimate fair values of its leasehold improvements and right-of-use assets, including leasehold improvements in the process of construction, and a market approach to estimate the fair value of its furniture, fixtures and laboratory and manufacturing equipment. These represented level 3 nonrecurring fair value measurements. Based on this analysis, the Company concluded the fair values of certain lease related long-lived assets were lower than their net book values due to declines in the market prices for leases. The Company recognized pre-tax long-lived asset impairment charges of $0.1 million on the right-of-use assets and $0.2 million on the related leasehold improvements and construction-in-progress during the three months ended December 31, 2023. The Company will continue to assess whether its long-lived assets are impaired in future periods. As the Company finalizes and implements its plans related to cost reductions and liquidity preservation, it is reasonably possible that additional impairment charges will be recognized if the Company changes how it uses various long-lived assets or elects to dispose of them, and the cash flows associated with these assets become separately identifiable. In this case, such assets will be tested for impairment separately from the remaining long-lived assets of the Company.