Commitments and contingencies	9 Months Ended
Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	8. Commitments and contingencies Operating lease commitments On June 3, 2016, the Company entered into a manufacturing agreement for the future commercial production of the Company’s Lenti-D and LentiGlobin product candidates with a contract manufacturing organization. Under this 12 year agreement, the contract manufacturing organization will complete the design, construction, validation and process validation of the leased suites prior to anticipated commercial launch of the product candidates. During construction, the Company was required to pay $12.5 million upon the achievement of certain contractual milestones, and may pay up to $8.0 million in additional contractual milestones if the Company elects its option to lease additional suites. The Company paid $5.0 million for the achievement of the first and second contractual milestones during 2016 and paid $5.5 million for the third and fourth contractual milestones achieved during 2017. In March 2018, $1.5 million of the possible $2.0 million related to the fifth contractual milestone was achieved and was paid in the second quarter of 2018. Given that construction was completed in March 2018, beginning in April 2018 the Company will pay $5.1 million per year in fixed suite fees as well as certain fixed labor, raw materials, testing and shipping costs for manufacturing services, and may pay additional suite fees if it elects its option to reserve or lease additional suites. The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees. The Company concluded that this agreement contains an embedded lease as the suites are designated for the Company’s exclusive use during the term of the agreement. The Company concluded that it is not the deemed owner during construction nor is it a capital lease under ASC 840-10, Leases – Overall. As a result, the Company accounts for the agreement as an operating lease and expenses the rental payments on a straight-line basis over the non-cancellable term of the embedded lease. On November 18, 2016, the Company entered into an agreement for future clinical and commercial production of the Company’s LentiGlobin and Lenti-D gene therapy drug products with a contract manufacturing organization at an existing facility. The term of the agreement is five years with a three year renewal at the mutual option of each party. Under the agreement, the Company is required to pay an up-front fee of €3.0 million, €2.0 million of which was paid in the fourth quarter of 2016 and €1.0 million of which was paid in the third quarter of 2018, and annual maintenance and production fees of up to €9.8 million, depending on its production needs. The Company may terminate this agreement with twelve months’ notice and a one-time termination fee. The Company concluded that this agreement contains an embedded lease as the clean rooms are designated for the Company’s exclusive use during the term of the agreement and determined that it is not a capital lease under ASC 840-10, Leases – Overall. As a result, the Company accounts for the agreement as an operating lease and expenses the rental payments on a straight-line basis over the non-cancellable term of the embedded lease. 60 Binney Street Lease commitments On September 21, 2015, the Company entered into a lease agreement for office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”) for its corporate headquarters with a term through March 31, 2027. Although the Company does not legally own the premises, it is deemed to be the owner of the building for accounting purposes because the Company was involved in the construction project, including having responsibility to pay for a portion of the costs of finish work and mechanical, electrical, and plumbing elements of the Building during the construction period. Accordingly, construction costs that were incurred by the landlord directly or indirectly through reimbursement to the Company as part of its tenant improvement allowance have been recorded as an asset in property, plant and equipment, net with a related financing obligation in accrued expenses and other current liabilities and financing lease obligation, net of current portion on the Company’s condensed consolidated balance sheets. The Company evaluated the 60 Binney Street Lease upon occupancy on March 27, 2017 and determined that the 60 Binney Street Lease did not meet the criteria for “sale-leaseback” treatment. This determination was based on, among other things, the Company's continuing involvement with the property in the form of non-recourse financing to the lessor. Accordingly, upon occupancy, the Company commenced depreciating the building over a useful life of 40 years and incurred interest expense related to the financing obligation of $3.9 million and $11.6 million for the three and nine months ended September 30, 2018, respectively. The Company incurred interest expense related to the financing obligation of $4.1 million and $7.8 million for the three and nine months ended September 30, 2017, respectively. The Company currently maintains a $13.8 million letter of credit as required under the terms of the lease. Subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease to $9.2 million over time. Contingent consideration related to business combinations On June 30, 2014, the Company acquired Pregenen. The Company may be required to make up to $120.0 million in remaining future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain clinical and commercial milestones related to the Pregenen technology, of which $20.1 million relates to clinical milestones and $99.9 million relates to commercial milestones. In accordance with accounting guidance for business combinations, contingent consideration liabilities are required to be recognized on the consolidated balance sheets at fair value. Estimating the fair value of contingent consideration requires the use of significant assumptions primarily relating to probabilities of successful achievement of certain clinical and commercial milestones, the expected timing in which these milestones will be achieved and discount rates. The use of different assumptions could result in materially different estimates of fair value. Please refer to Note 4, “Fair value measurements” for additional information. Other funding commitments The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2018 and December 31, 2017 or royalties on future sales of specified products. Additionally, the Company is party to various contracts with contract research organizations and contract manufacturers that generally provide for termination on notice, with the exact amounts due in the event of termination to be based on the timing of the termination and the terms of the agreement. In each of 2018 and 2019, the Company expects to make payments of approximately $4.0 million and $23.0 million, respectively, under an agreement with a contract manufacturer.

8. Commitments and contingencies

Operating lease commitments

On June 3, 2016, the Company entered into a manufacturing agreement for the future commercial production of the Company’s Lenti-D and LentiGlobin product candidates with a contract manufacturing organization. Under this 12 year agreement, the contract manufacturing organization will complete the design, construction, validation and process validation of the leased suites prior to anticipated commercial launch of the product candidates. During construction, the Company was required to pay $12.5 million upon the achievement of certain contractual milestones, and may pay up to $8.0 million in additional contractual milestones if the Company elects its option to lease additional suites. The Company paid $5.0 million for the achievement of the first and second contractual milestones during 2016 and paid $5.5 million for the third and fourth contractual milestones achieved during 2017. In March 2018, $1.5 million of the possible $2.0 million related to the fifth contractual milestone was achieved and was paid in the second quarter of 2018. Given that construction was completed in March 2018, beginning in April 2018 the Company will pay $5.1 million per year in fixed suite fees as well as certain fixed labor, raw materials, testing and shipping costs for manufacturing services, and may pay additional suite fees if it elects its option to reserve or lease additional suites. The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees. The Company concluded that this agreement contains an embedded lease as the suites are designated for the Company’s exclusive use during the term of the agreement. The Company concluded that it is not the deemed owner during construction nor is it a capital lease under ASC 840-10, Leases – Overall. As a result, the Company accounts for the agreement as an operating lease and expenses the rental payments on a straight-line basis over the non-cancellable term of the embedded lease.

On November 18, 2016, the Company entered into an agreement for future clinical and commercial production of the Company’s LentiGlobin and Lenti-D gene therapy drug products with a contract manufacturing organization at an existing facility. The term of the agreement is five years with a three year renewal at the mutual option of each party. Under the agreement, the Company is required to pay an up-front fee of €3.0 million, €2.0 million of which was paid in the fourth quarter of 2016 and €1.0 million of which was paid in the third quarter of 2018, and annual maintenance and production fees of up to €9.8 million, depending on its production needs. The Company may terminate this agreement with twelve months’ notice and a one-time termination fee. The Company concluded that this agreement contains an embedded lease as the clean rooms are designated for the Company’s exclusive use during the term of the agreement and determined that it is not a capital lease under ASC 840-10, Leases – Overall. As a result, the Company accounts for the agreement as an operating lease and expenses the rental payments on a straight-line basis over the non-cancellable term of the embedded lease.

60 Binney Street Lease commitments

On September 21, 2015, the Company entered into a lease agreement for office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”) for its corporate headquarters with a term through March 31, 2027. Although the Company does not legally own the premises, it is deemed to be the owner of the building for accounting purposes because the Company was involved in the construction project, including having responsibility to pay for a portion of the costs of finish work and mechanical, electrical, and plumbing elements of the Building during the construction period. Accordingly, construction costs that were incurred by the landlord directly or indirectly through reimbursement to the Company as part of its tenant improvement allowance have been recorded as an asset in property, plant and equipment, net with a related financing obligation in accrued expenses and other current liabilities and financing lease obligation, net of current portion on the Company’s condensed consolidated balance sheets.

The Company evaluated the 60 Binney Street Lease upon occupancy on March 27, 2017 and determined that the 60 Binney Street Lease did not meet the criteria for “sale-leaseback” treatment. This determination was based on, among other things, the Company's continuing involvement with the property in the form of non-recourse financing to the lessor. Accordingly, upon occupancy, the Company commenced depreciating the building over a useful life of 40 years and incurred interest expense related to the financing obligation of $3.9 million and $11.6 million for the three and nine months ended September 30, 2018, respectively. The Company incurred interest expense related to the financing obligation of $4.1 million and $7.8 million for the three and nine months ended September 30, 2017, respectively. The Company currently maintains a $13.8 million letter of credit as required under the terms of the lease. Subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease to $9.2 million over time.

Contingent consideration related to business combinations

On June 30, 2014, the Company acquired Pregenen. The Company may be required to make up to $120.0 million in remaining future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain clinical and commercial milestones related to the Pregenen technology, of which $20.1 million relates to clinical milestones and $99.9 million relates to commercial milestones. In accordance with accounting guidance for business combinations, contingent consideration liabilities are required to be recognized on the consolidated balance sheets at fair value. Estimating the fair value of contingent consideration requires the use of significant assumptions primarily relating to probabilities of successful achievement of certain clinical and commercial milestones, the expected timing in which these milestones will be achieved and discount rates. The use of different assumptions could result in materially different estimates of fair value. Please refer to Note 4, “Fair value measurements” for additional information.

Other funding commitments

The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2018 and December 31, 2017 or royalties on future sales of specified products. Additionally, the Company is party to various contracts with contract research organizations and contract manufacturers that generally provide for termination on notice, with the exact amounts due in the event of termination to be based on the timing of the termination and the terms of the agreement. In each of 2018 and 2019, the Company expects to make payments of approximately $4.0 million and $23.0 million, respectively, under an agreement with a contract manufacturer.