IMPAIRMENT OF GOODWILL, INDEFINITE-LIVED INTANGIBLE ASSETS AND LONG-LIVED ASSETS AND WRITE-DOWN OF ASSETS HELD FOR SALE	12 Months Ended
	Dec. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
IMPAIRMENT OF GOODWILL, INDEFINITE-LIVED INTANGIBLE ASSETS AND LONG-LIVED ASSETS AND WRITE-DOWN OF ASSETS HELD FOR SALE	IMPAIRMENT OF GOODWILL, INDEFINITE-LIVED INTANGIBLE ASSETS AND LONG-LIVED ASSETS AND WRITE-DOWN OF ASSETS HELD FOR SALE Year ended December 31, 2024 During the three months ended March 31, 2024, the Company’s Board of Directors approved the wind-down of research and development activities in France and corresponding reduction in workforce, including closure of the Company’s cell therapy manufacturing facility and research labs in Valbonne, France (the “France Restructuring”). The Company concluded its equipment, furniture and fixtures located in France met the held for sale criteria as of March 31, 2024. The Company wrote down the carrying value of these assets to their estimated fair value of $1.0 million, net of the estimated costs to sell, recognizing a loss of $1.8 million. An additional loss of $0.1 million was recorded during the three months ended June 30, 2024. The fair value measurements represent Level 3 nonrecurring fair value measurements. The loss is included in impairment of long-lived assets in the accompanying Consolidated Statements of Operations. The Company sold assets held for sale during the three months ended December 31, 2024, and recognized a gain of $1.0 million included in general and administrative expenses. See Note 10 – Restructuring Charges, for the additional effects of the France Restructuring. During the three months ended March 31, 2024, the Company also initiated actions to commence the closure of its facility in Brisbane, California. The Company expects to close this facility in the near future. In connection with the changes in the manner in which the right-of-use assets and leasehold improvements related to the Company’s Brisbane, California and Valbonne, France facilities are used, costs incurred to cease use of these assets, the France Restructuring, and the Company’s activities to market these facilities for sublease, the Company concluded the identifiable operations and cash flows of these assets were now largely independent of the operations and the cash flows of each other, as well as of the remainder of the Company. Accordingly, the Company assessed impairment for each of these asset groups separately and concluded that the carrying values of Brisbane, California and Valbonne, France facilities asset groups were not recoverable. The Company proceeded to determine their fair values using a discounted cash flow method, which represents a Level 3 nonrecurring fair value measurement. As a result, the Company recognized pre-tax long-lived asset impairment charges of $2.0 million on the right-of-use assets and $0.5 million on the related leasehold improvements during the three months ended March 31, 2024. Additional impairment charges of $0.9 million on the right-of-use assets and $0.2 million on the related leasehold improvements were recognized during the three months ended June 30, 2024, triggered by the ongoing wind-down of the France research and development activities and a decline in the market rates for facility subleases in Brisbane, California. The Company will continue to assess whether its long-lived assets are impaired in future periods. As the Company finalizes the closure of its Brisbane, California facility, it is reasonably possible that additional impairment charges will be recognized, for example, if sublease rates of leased facilities are less than those estimated. Year ended December 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, and restructuring of operations the Company performed an impairment assessment of goodwill, indefinite-lived intangible assets, and other long-lived assets. The Company operated 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 represented 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 year ended December 31, 2023. As a result, the goodwill was fully impaired as of December 31, 2023. Throughout the year ended December 31, 2023, the Company determined all of its long-lived assets represented one asset group for purposes of long-lived asset impairment assessment. At each of the periods ended March 31, June 30, September 30 and December 31, 2023, the Company concluded that the carrying value of the asset group was not recoverable as it exceeded the future undiscounted cash flows the assets were 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 and 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 these analyses, the Company recognized pre-tax long-lived asset impairment charges of $28.9 million on the right-of-use assets, $18.7 million on the related leasehold improvements and $4.4 million on construction-in-progress, and $13.5 million on furniture, fixtures, and laboratory and manufacturing equipment during the year ended December 31, 2023. 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 $51.3 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 was recognized during the year ended December 31, 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.