Commitments and contingencies	9 Months Ended
Sep. 30, 2014
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	5. Commitments and contingencies On June 3, 2013, the Company entered into a nine-year building lease for approximately 43,600 square feet of space for its new corporate headquarters at 150 Second Street, Cambridge, Massachusetts, which commenced in December 2013. The lease originally had monthly lease payments of $209 the first 12 months with annual rent escalations thereafter and provided a rent abatement of $209 per month for the first six months. The Company has the option to extend this lease by an additional five years. As the Company obtained access to the newly leased space on July 22, 2013 in order to begin the build-out, this is considered the lease commencement date for accounting purposes, thus rent expense began on this date and is recognized on a straight-line basis over the term of the lease. In addition, the lease provided a contribution from the landlord towards the initial build-out of the space of up to $6,538. The Company capitalizes the leasehold improvements as property and equipment and records the landlord incentive payments received as deferred rent and amortizes these amounts as reductions to rent expense over the lease term. In accordance with the lease, the Company entered into a cash-collateralized irrevocable standby letter of credit in the amount of $1,253, naming the landlord as beneficiary. This letter of credit was reduced to $1,044 during the third quarter and may be further reduced to $835 and $627 upon the first and second anniversaries of the rent commencement date, respectively. The Company relocated into its new corporate headquarters in December 2013 and ceased use of its former facility during the first quarter of 2014. During the first quarter of 2014, the Company fully subleased its former facility, which decreased the rent abatement available under its new lease. The lease for the Company’s former headquarters, located at 840 Memorial Drive, Cambridge, Massachusetts, expires on March 31, 2015. On June 9, 2014, the Company amended its lease agreement to add an additional 9,869 square feet of space for its corporate headquarters at 150 Second Street, Cambridge, Massachusetts. The expansion increases the monthly lease payments by $47 beginning in September 2014 with rent escalations thereafter. In addition, the lease amendment provides a contribution from the landlord towards the build-out of the additional space of up to $1,234. The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2014 and December 31, 2013 or royalties on future sales of specified products. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to these agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with claims by any third party with respect to the Company’s products or business activities. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements. The Company’s wholly-owned subsidiary bluebird bio France – SARL participates in the French Crédit d’Impôt Recherche (“CIR”) program which allows companies to monetize up to 30% of eligible research expenses. The Company received aggregate reimbursement of $807 related to years 2010 through 2012. The Company recognized these amounts as reductions to research and development expense in the periods incurred. The years 2010 through 2013 are open and subject to examination. The Company has not yet applied for $699 related to the nine months ended September 30, 2014 which are classified as current assets within the condensed consolidated balance sheets as of September 30, 2014. On June 30, 2014, the Company acquired Pregenen. The Company may be required to make up to an additional $135,000 in future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain preclinical, clinical and commercial milestones related to the Pregenen technology, of which $15,000 relates to preclinical milestones, $20,100 relates to clinical milestones and $99,900 relates to commercial milestones. In accordance with accounting for business combinations guidance, contingent consideration liabilities are required to be recognized on the condensed 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 preclinical, 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. See Note 4, “Fair value measurements,” and Note 7, “Business combinations,” for additional information.

5. Commitments and contingencies

On June 3, 2013, the Company entered into a nine-year building lease for approximately 43,600 square feet of space for its new corporate headquarters at 150 Second Street, Cambridge, Massachusetts, which commenced in December 2013. The lease originally had monthly lease payments of $209 the first 12 months with annual rent escalations thereafter and provided a rent abatement of $209 per month for the first six months. The Company has the option to extend this lease by an additional five years. As the Company obtained access to the newly leased space on July 22, 2013 in order to begin the build-out, this is considered the lease commencement date for accounting purposes, thus rent expense began on this date and is recognized on a straight-line basis over the term of the lease. In addition, the lease provided a contribution from the landlord towards the initial build-out of the space of up to $6,538. The Company capitalizes the leasehold improvements as property and equipment and records the landlord incentive payments received as deferred rent and amortizes these amounts as reductions to rent expense over the lease term. In accordance with the lease, the Company entered into a cash-collateralized irrevocable standby letter of credit in the amount of $1,253, naming the landlord as beneficiary. This letter of credit was reduced to $1,044 during the third quarter and may be further reduced to $835 and $627 upon the first and second anniversaries of the rent commencement date, respectively. The Company relocated into its new corporate headquarters in December 2013 and ceased use of its former facility during the first quarter of 2014. During the first quarter of 2014, the Company fully subleased its former facility, which decreased the rent abatement available under its new lease. The lease for the Company’s former headquarters, located at 840 Memorial Drive, Cambridge, Massachusetts, expires on March 31, 2015.

On June 9, 2014, the Company amended its lease agreement to add an additional 9,869 square feet of space for its corporate headquarters at 150 Second Street, Cambridge, Massachusetts. The expansion increases the monthly lease payments by $47 beginning in September 2014 with rent escalations thereafter. In addition, the lease amendment provides a contribution from the landlord towards the build-out of the additional space of up to $1,234.

The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2014 and December 31, 2013 or royalties on future sales of specified products.

The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to these agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with claims by any third party with respect to the Company’s products or business activities. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

The Company’s wholly-owned subsidiary bluebird bio France – SARL participates in the French Crédit d’Impôt Recherche (“CIR”) program which allows companies to monetize up to 30% of eligible research expenses. The Company received aggregate reimbursement of $807 related to years 2010 through 2012. The Company recognized these amounts as reductions to research and development expense in the periods incurred. The years 2010 through 2013 are open and subject to examination. The Company has not yet applied for $699 related to the nine months ended September 30, 2014 which are classified as current assets within the condensed consolidated balance sheets as of September 30, 2014.

On June 30, 2014, the Company acquired Pregenen. The Company may be required to make up to an additional $135,000 in future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain preclinical, clinical and commercial milestones related to the Pregenen technology, of which $15,000 relates to preclinical milestones, $20,100 relates to clinical milestones and $99,900 relates to commercial milestones. In accordance with accounting for business combinations guidance, contingent consideration liabilities are required to be recognized on the condensed 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 preclinical, 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. See Note 4, “Fair value measurements,” and Note 7, “Business combinations,” for additional information.