LICENSE AND COLLABORATION AGREEMENTS	12 Months Ended
LICENSE AND COLLABORATION AGREEMENTS	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
LICENSE AND COLLABORATION AGREEMENTS	LICENSE AND COLLABORATION AGREEMENTSIn October 2019, the Company entered into a worldwide, exclusive licensing agreement with a third party for tralesinidase alfa (formerly referred to as BMN 250), an investigational enzyme replacement therapy to treat Sanfilippo Syndrome Type B. In consideration, the Company received an upfront payment of $3.0 million, a minority 15% equity ownership interest in the licensee, and is entitled to receive royalties on net sales of tralesinidase alfa and milestone payments if certain development, regulatory and sales milestones are met by the licensee. Subsequently, the third party licensee raised additional funding and issued the Company incremental shares to maintain its 15% minority interest. The Company evaluated the design and purpose of the third-party licensee and determined that it is a variable interest entity (VIE), as the equity-at-risk is insufficient to support the licensee’s operations. The Company has concluded that it is not the primary beneficiary of the VIE as the Company does not have the power to direct the activities of the VIE that most significantly impact its performance. The Company is accounting for the minority equity investment at cost, less impairment, if any, adjusted for observable price changes, as it does not exercise significant influence over the operations of the licensee. Other than providing the licensee with specified transition services, the Company has no other involvement with the operations of the VIE as of December 31, 2022. As a result, the Company's loss exposure is limited to the value of the equity investment of $12.6 million which is included in Other Assets on the Company’s Consolidated Balance Sheets as of December 31, 2022. In July 2017, the Company executed a license agreement with Sarepta Therapeutics (Sarepta) that provides Sarepta with global exclusive rights to the Company’s Duchenne muscular dystrophy (DMD) patent estate for EXONDYS 51 and all future exon-skipping products. Under the license agreement, Sarepta pays the Company royalties and may pay the Company certain milestone payments for exons 51, 45, 53 and possibly other exon-skipping products. In the fourth quarter of 2021, the Company and Sarepta amended the license agreement to, among other things, make the license co-exclusive at a future date and reduce future royalty rates. On October 1, 2015, the Company entered into an agreement with Ares Trading S.A. (Merck Serono) under which the Company acquired all global rights to KUVAN and PALYNZIQ from Merck Serono, with the exception of KUVAN in Japan. Previously, the Company had exclusive rights to KUVAN in the U.S. and Canada and PALYNZIQ in the U.S. and Japan. Pursuant to the amended and restated KUVAN Agreement, if future sales milestones were met, the Company was obligated to pay Merck Serono up to a maximum of €60.0 million, all of which were met as of December 31, 2022, €15.0 million of which was unpaid as of December 31, 2022. Pursuant to the Pegvaliase Agreement, the Company also paid Merck Serono €125.0 million in cash when the PALYNZIQ development milestones were achieved. In October 2012, the Company licensed to Catalyst Pharmaceutical Partners, Inc. (Catalyst) the North American rights to develop and market FIRDAPSE, the Company's former commercial product for the treatment of Lambert-Eaton myasthenic syndrome. In exchange for the North American rights to FIRDAPSE, commencing in the first quarter of 2019 the Company receives royalties of 7% to 10% on net product sales of FIRDAPSE in North America. In January 2020, the Company completed the sale of worldwide rights to FIRDAPSE to a third party. The Company retained the rights to receive the royalties from Catalyst. See Note 1 to these Consolidated Financial Statements for further information about the FIRDAPSE sale. In September 2007, the Company licensed to Asubio Pharma Co., Ltd. (a subsidiary of Daiichi Sankyo) exclusive rights to data and intellectual property contained in the KUVAN New Drug Application. The Company receives royalties on net sales of the product in Japan. The Company is engaged in R&D collaborations with various other entities. These provide for sponsorship of R&D by the Company and may also provide for exclusive royalty-bearing intellectual property licenses or rights of first negotiation regarding licenses to intellectual property development under the collaborations. Typically, these agreements can be terminated for cause by either party upon written notice.