Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies License Agreement The Company has entered into an exclusive license agreement with UCLB which has subsequently been amended and restated (see Note 8). In connection with the UCLB license agreement, the Company is required to make annual license payments and may be required to make payments upon the achievement of specified milestones. The Company has estimated the probability of the Company achieving each potential milestone in accordance with ASC 450, Contingencies. The Company concluded that, as of September 30, 2020 there was a $0.6 million milestone related to the receipt of the clinical data for its AUTO1 program, the achievement of which was considered probable, and accordingly, the Company has accrued a liability for the expected milestone payment to UCLB of $0.6 million as of September 30, 2020. As of September 30, 2020, there were no other milestones for which the likelihood of achievement was probable. Legal Proceedings From time to time, the Company may be a party to litigation or subject to claims incident to the ordinary course of business. Regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors. The Company was not a party to any litigation and did not have contingency reserves established for any liabilities as of September 30, 2020 and December 31, 2019. Leases The Company leases certain office space, laboratory space, and equipment. At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. The Company’s corporate headquarters are located in London, United Kingdom. The Company leases space at this location from Imperial (Forest House) Limited under a ten-year lease, the term of which commenced in September 2015. The lease included an option for the Company to lease additional space within a 15-month period, which the Company exercised in October 2016. The exercise of the option resulted in a separate new lease agreement with a concurrent term through September 2025. The landlord exercised its option to give notice in September 2020 to terminate the Forest House lease and pay the Company a break-lease payment fee in September 2021. The Company recorded a $1.3 million gain upon the notice of lease termination and lease incentive receivable of $1.2 million as of September 30, 2020. In September 2017, the Company executed an arrangement with Catapult Limited to lease a manufacturing suite at the Cell and Gene Therapy Catapult manufacturing center in Stevenage, United Kingdom for a term through May 2021, at which time the Company has the option to renew or terminate the lease. The lease related to this facility is classified as an operating lease. The lease had a six-month rent-free period. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. In December 2018, the Company executed an additional lease arrangement for additional manufacturing space for a term through September 2023, at which time the Company has the option to renew or terminate the lease. In June 2018, the Company signed a binding letter of intent to enter into a lease for office and laboratory space in White City, London. The letter of intent required the Company to enter into a ten-year lease provided that the landlord completed the required leasehold improvements described in the agreement. The leasehold improvements were completed, and the lease commenced in January 2019. The Company has the option to terminate the lease in November 2026. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. The lease agreement includes an option to lease additional space. In September 2018, the Company signed a binding letter of intent to enter into a lease for manufacturing space in Enfield, United Kingdom. The letter of intent required the Company to enter into a 15-year lease provided that the landlord completed the required leasehold improvements described in the agreement. The Company executed lease agreements for three manufacturing space units, each for 15-year lease terms. The leases commenced in February 2019 with option to terminate the lease in February 2029. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. In December 2019, the Company discontinued the fit-out of the manufacturing facility, expensed $4.1 million of leasehold improvements from assets under construction in 2019, and reduced the right-of-use asset and lease liability based on the contractual option termination date. The Company is actively seeking to sub-lease or assign the lease arrangement to a third party. The Company completed an asset impairment analysis of the right of use lease asset and concluded the undiscounted cash flows exceeded the carrying value as of September 30, 2020, such that no impairment needed to be recorded. In October 2018, the Company executed an agreement to sublease office space in Rockville, Maryland for a term through October 2021. The Company then terminated the sublease in February 2020 and immediately entered into a five-year lease for the same space with the landlord. As a result of the sublease termination, the Company recognized a $0.2 million gain in other income (expense) in the first quarter of 2020. The lease related to this facility is classified as an operating lease. The Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. In January 2019, the Company executed a lease agreement to lease additional office and manufacturing space in Rockville, Maryland. The lease agreement required the Company to enter into a lease provided that the landlord completes the required leasehold improvements described in the agreement. The lease commenced in August 2020 for a term through June 2036. The lease related to this facility is classified as an operating lease. The Company has capitalized $2.2 million in leasehold improvements as assets under construction as of September 30, 2020. In May 2020, the Company executed an arrangement with Catapult Limited to lease a manufacturing suite at the Cell and Gene Therapy Catapult manufacturing center in Stevenage, United Kingdom for a term through April 2024. The lease related to this facility is classified as an operating lease. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability