Acquisitions	12 Months Ended
Acquisitions	Dec. 31, 2019
Business Combinations [Abstract]
Acquisitions	Acquisitions Acquisition of Celenex In September 2018, the Company expanded its pipeline by acquiring the rights and related intellectual property of ten gene therapy programs through its acquisition of Celenex, Inc., ("Celenex"). Celenex is a private, clinical stage gene therapy company whose lead programs are ten gene therapy programs including CLN6 and CLN3, which are in clinical stage, and several programs in pre-clinical stage. Pursuant to the terms of the agreement, the Company acquired Celenex for cash consideration of $100 million. The Company has also agreed to pay up to an additional $15 million in connection with the achievement of certain development milestones, $262 million in connection with the achievement of certain regulatory approval milestones across multiple programs and up to $75 million in tiered sales milestone payments. Celenex has an exclusive license agreement with Nationwide Children’s Hospital ("NCH"). Under this license agreement, NCH is eligible to receive development and sales-based milestones of up to $7.8 million for each product. The Company evaluated the Celenex transaction and concluded that the transaction did not meet the definition of a business and was an asset acquisition. Given the fact that the license has no alternative future use, the $100.0 million upfront payment was expensed to research and development expense in the Consolidated Statements of Operations for the year ended December 31, 2018. Acquisition of MiaMed, Inc. In July 2016, the Company entered into an Agreement and Plan of Merger (the "MiaMed Agreement") with MiaMed, Inc., ("MiaMed"). MiaMed is a pre-clinical biotechnology company focused on developing protein replacement therapy for CDD and related diseases. Under the terms of the MiaMed Agreement, the former holders of MiaMed's capital stock received an aggregate of $6.5 million, comprised of (i) approximately $1.8 million in cash (plus MiaMed's cash and cash equivalents at closing and less any of MiaMed's unpaid third-party fees and expenses related to the transaction), and (ii) 825,603 shares of the Company's common stock. In addition, the Company also agreed to pay up to an additional $83.0 million in connection with the achievement of certain clinical, regulatory, and commercial milestones, for a potential aggregate deal value of $89.5 million. The Company evaluated the transaction and concluded that it only acquired inputs and did not acquire any processes. The Company will need to develop its own processes in order to produce an output. Therefore, the Company accounted for the transaction as an asset acquisition and accordingly $6.5 million was expensed to research and development expense in the Consolidated Statements of Operation for the year ended December 31, 2016. Acquisition of Scioderm, Inc. In September 2015, the Company acquired Scioderm Inc., ("Scioderm"), a privately-held biopharmaceutical company focused on developing innovative therapies for treating the rare disease, Epidermolysis Bullosa ("EB"). The acquisition potentially leveraged the Scioderm development team's EB expertise with the Company's global clinical infrastructure to advance SD-101toward regulatory approvals and the Company's commercial, patient advocacy, and medical affairs infrastructure to support a successful global launch. The acquisition of Scioderm was accounted for as a purchase of a business in accordance with ASC 805 Business Combinations. At the end of the first quarter of 2017, the Company achieved 100% enrollment in the Phase 3 clinical study of SD-101 and the milestone payment of $10 million due for this event, was paid in April 2017. On September 13, 2017, the Company reported that top-line data from the randomized, double-blind, placebo-controlled Phase 3 clinical study ("ESSENCE, SD-5") to assess the efficacy and safety of the novel topical wound-healing agent SD-101 did not meet the primary endpoints or secondary endpoints in participants with EB. Based on these top-line data, the Company has no current plans to invest in any additional clinical studies or commercial preparation activities for SD-101. This event led the Company to assess the carrying amount of the program's tangible and intangible assets against their respective fair values. Based on the assessment, the Company recognized a loss on impairment of intangible assets in the amount of $463.7 million and $1.7 million in fixed assets recorded within loss on impairment of assets within the Consolidated Statements of Operations. Since the study did not meet the primary and secondary endpoints, the Company concluded that they will not make the potential milestone payments indicated in the Asset Purchase Agreement to the former Scioderm holders. Accordingly, the Company recognized a gain of $254.7 million in changes in fair value of contingent consideration payable in the third quarter of 2017, in order to decrease the liability to zero. The Company also recognized $0.4 million in selling, general, and administrative costs and $8.1 million in research and development expenses related to the wind-down of operations for the Phase 3 ESSENCE study and ongoing extension studies SD-004 and SD-006, as well as income tax benefit of $164.7 million due to the reduction of the deferred tax liability related to Scioderm IPR&D, in the Consolidated Statements of Operations in the third quarter of 2017. Acquisition of Callidus Biopharma, Inc. In November 2013, the Company acquired Callidus Biopharma, Inc., ("Callidus"), a privately-held biologics company focused on developing best-in-class ERTs for LDs with its lead ERT ATB200 for Pompe disease in late preclinical development. The fair value of the contingent acquisition consideration payments was estimated by applying a probability-based income approach utilizing an appropriate discount rate. Key assumptions include discount rate and various probability factors. This estimation was based on significant inputs that are not observable in the market, referred to as Level 3 inputs. Some of the more significant assumptions used in the valuation include (i) the probability and timing related to the achievement of certain developmental milestones and (ii) and the discount rate. See "— Note 11. Assets and Liabilities Measured at Fair Value", for additional discussion regarding fair value measurements of the contingent acquisition consideration payable. The Company determined the fair value of the contingent consideration to be $22.7 million at December 31, 2019, which is payable beyond the next twelve months, resulting in an increase in the contingent consideration payable and related expense of $3.3 million in the year ended December 31, 2019. The expense is recorded in the Consolidated Statement of Operations within the changes in fair value of contingent consideration line item. During the fourth quarter of 2018, the Company reached a clinical milestone for Callidus, which was the dosing of the first patient in a Phase 3 study. The milestone payment for this event was $9.0 million which was paid in the Company's stock during the first quarter of 2019. For further information, see "— Note 4. Goodwill and Intangible Assets."