Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2019
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS In February 2020, the Company entered into a global licensing collaboration agreement with Biogen MA, Inc. and its affiliate, Biogen International GmbH, for the research, development and commercialization of gene regulation therapies for the treatment of neurological diseases. The companies plan to leverage the Company’s proprietary ZFP technology delivered via AAV to modulate expression of key genes involved in neurological diseases. Under the Biogen collaboration agreement, the Company will grant to Biogen an exclusive, royalty bearing and worldwide license, under its relevant patents and know-how, to develop, manufacture and commercialize certain ZFP and/or AAV-based products directed to up to 12 neurological disease gene targets selected by Biogen. Biogen has already selected three of these: ST-501 for tauopathies including Alzheimer’s disease, ST-502 for synucleinopathies including Parkinson’s disease, and a third undisclosed neuromuscular disease target. Biogen has exclusive rights to nominate up to nine additional targets over a target selection period of five years. For each gene target selected by Biogen, the Company will perform early research activities, costs for which will be shared by the companies, aimed at the development of the combination of proprietary CNS delivery vectors and ZFP-TFs (or potential other ZFP products) targeting therapeutically relevant genes. Biogen will then assume responsibility and costs for the IND-enabling studies, clinical development, related regulatory interactions, and global commercialization. The Company will be responsible for GMP manufacturing activities for the initial clinical trials for the first three products of the collaboration and plans to leverage its in-house manufacturing capacity. Biogen will assume responsibility for GMP manufacturing activities beyond the first clinical trial for each of the first three products. Subject to certain exceptions set forth in the Biogen collaboration agreement, the Company will be prohibited from developing, manufacturing or commercializing any therapeutic product directed to the targets selected by Biogen. After the Biogen collaboration agreement becomes effective, Biogen will pay the Company an upfront payment of $125.0 million. The Company is also eligible to receive research, development, regulatory and commercial milestone payments that could total up to approximately $2.37 billion if Biogen selects all of the targets allowed under the agreement and all the specified milestones set forth in the agreement are achieved, which includes up to $925.0 million in pre-approval milestone payments and up to $1,445.0 million in first commercial sale and other sales-based milestone payments. In addition, the Company will also be eligible to receive tiered high single-digit to sub-teen royalties on potential net commercial sales of licensed products arising from the collaboration. These royalty payments will be subject to reduction due to patent expiration, entry of biosimilar products to the market and payments made under certain licenses for third-party intellectual property. The Biogen collaboration agreement will continue, on a product-by-product and country-by-country basis, until the expiration of the applicable royalty term. Biogen has the right to terminate the collaboration agreement, in its entirety or on target-by-target basis, for any reason after a specified notice period. Each party has the right to terminate this agreement on account of the other party’s bankruptcy or material, uncured breach. In addition, the Company may terminate the collaboration agreement if Biogen challenges any patents licensed by the Company to Biogen. Concurrently with the execution of the collaboration agreement, the Company also entered into a stock purchase agreement with Biogen MA, Inc., pursuant to which Biogen will purchase 24,420,157 shares of the Company’s common stock (the “Biogen Shares”), at a price per share of $9.2137, for an aggregate purchase price of $225.0 million. Pursuant to the terms of the stock purchase agreement, Biogen has agreed not to, without the Company’s prior written and subject to specified conditions and exceptions, directly or indirectly acquire shares of the Company’s outstanding common stock, seek or propose a tender or exchange offer or merger between the parties, solicit proxies or consents with respect to any matter, or undertake other specified actions related to the potential acquisition of additional equity interests in the Company. Such standstill restrictions expire on the earlier of the three-year anniversary of the effectiveness of the Biogen collaboration agreement and the date that Biogen beneficially owns less than 5% of the Company’s common stock. The stock purchase agreement also provides that until the first anniversary of the effectiveness of the Biogen collaboration agreement, Biogen will hold and not sell any of the Biogen Shares and from the first anniversary through the second anniversary, Biogen will hold and not sell at least 50% of the Biogen Shares, in addition to being subject to certain volume limitations. The stock purchase agreement further provides that, subject to certain limitations, upon Biogen’s request, the Company will register for resale any of the Biogen Shares on a registration statement to be filed with the SEC, until such time as all remaining Biogen Shares may be sold pursuant to Rule 144 promulgated under the Securities Act during any 90-day period. In addition, Biogen has agreed that, excluding specified extraordinary matters, it will vote the Biogen Shares in accordance with the Company’s recommendation and has granted the Company an irrevocable proxy with respect to the foregoing. Such voting provisions expire on the earlier of (i) the two-year anniversary of the effectiveness of the Biogen collaboration agreement, (ii) the date that Biogen beneficially owns less than 5% of the Company’s common stock and (iii) the date the Biogen collaboration agreement is terminated; provided, however, that in no event shall such expiration date be prior to the one-year anniversary of the effectiveness of the Biogen collaboration agreement. The consummation of the transactions under each of the Biogen collaboration agreement and the stock purchase agreement is subject to the satisfaction of customary closing conditions, including the expiration or termination of the applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended.