bluebird bio (BLUE) 10-Q 6 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 28, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	BLUE
Entity Registrant Name	bluebird bio, Inc.
Entity Central Index Key	1293971
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		32,829,440

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (Unaudited) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$133,174	$347,845
Marketable securities	201,710	125,710
Deferred tax assets	1,913	1,913
Prepaid expenses and other current assets	4,039	4,521
Total current assets	340,836	479,989
Marketable securities	134,430	18,448
Property and equipment, net	16,850	15,740
Intangible assets, net	27,278	28,219
Goodwill	13,128	13,128
Restricted cash and other non-current assets	1,294	1,215
Total assets	533,816	556,739
Current liabilities:
Accounts payable	2,551	2,954
Accrued expenses and other current liabilities	15,129	14,649
Deferred revenue, current portion	24,333	25,375
Total current liabilities	42,013	42,978
Deferred rent, net of current portion	8,456	8,674
Deferred revenue, net of current portion		5,302
Contingent consideration, net of current portion	6,670	6,321
Deferred tax liabilities	1,913	1,913
Other non-current liabilities	265	294
Total liabilities	59,317	65,482
Commitments and contingencies (Note 5)
Stockholders' equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at March 31, 2015 and December 31, 2014
Common stock, $0.01 par value, 125,000 shares authorized; 32,693 and 32,340 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	327	323
Additional paid-in capital	646,466	638,389
Accumulated other comprehensive loss	-123	-71
Accumulated deficit	-172,171	-147,384
Total stockholders' equity	474,499	491,257
Total liabilities and stockholders' equity	$533,816	$556,739

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Statement Of Financial Position [Abstract]
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	32,693,000	32,340,000
Common stock, shares outstanding	32,693,000	32,340,000

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenue:
Collaboration revenue	$6,344	$6,250
Research and license fees		85
Total revenue	6,344	6,335
Operating expenses:
Research and development	23,719	11,463
General and administrative	7,336	5,540
Change in fair value of contingent consideration	215
Total operating expenses	31,270	17,003
Loss from operations	-24,926	-10,668
Other income, net	139	59
Net loss	-24,787	-10,609
Other comprehensive loss:
Unrealized loss on available-for-sale securities, net of tax	-52
Comprehensive loss	($24,839)	($10,609)
Net loss per share - basic and diluted:	($0.76)	($0.44)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	32,558	24,148

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	($24,787)	($10,609)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,710	475
Stock-based compensation expense	5,432	2,493
Change in fair value of contingent consideration	215
Issuance of restricted common stock in exchange for consulting services to non-employees		42
Other non-cash items	130
Changes in operating assets and liabilities:
Prepaid expenses and other assets	-2	2,064
Accounts payable	-331	-2,388
Accrued expenses and other liabilities	88	1,738
Deferred revenue	-6,344	-6,335
Deferred rent	-270	586
Net cash used in operating activities	-24,159	-11,934
Investing activities
Purchase of property and equipment	-1,156	-2,517
Purchases of marketable securities	-223,020
Proceeds from maturities of marketable securities	31,200
Net cash used in investing activities	-192,976	-2,517
Financing activities
Proceeds from issuance of common stock	2,464	671
Net cash provided by financing activities	2,464	671
Decrease in cash and cash equivalents	-214,671	-13,780
Cash and cash equivalents at beginning of period	347,845	206,279
Cash and cash equivalents at end of period	133,174	192,499
Non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	$1,125	$947

Description_of_the_business

Description of the business	3 Months Ended
	Mar. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of the business	1. Description of the business
	bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company was formed to develop, manufacture and market therapies to safely and effectively deliver genes useful in the treatment of severe genetic and rare diseases and in the field of T cell-based immunotherapy. Since its inception, the Company has devoted substantially all of its resources to its research and development efforts relating to its product candidates, including activities to manufacture product candidates, conduct clinical studies of its product candidates, perform preclinical research to identify new product candidates and provide general and administrative support for these operations.

Summary_of_significant_account

Summary of significant accounting policies and basis of presentation	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of significant accounting policies and basis of presentation	2. Summary of significant accounting policies and basis of presentation
	Basis of presentation and principles of consolidation
	The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended March 31, 2015 and 2014.
	The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2014, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 25, 2015.
	The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Precision Genome Engineering, Inc. (“Pregenen”), bluebird bio France – SARL, bluebird bio Australia Pty Ltd. and bluebird bio Securities Corporation. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. All material long-lived assets of the Company reside in the United States.
	Summary of accounting policies
	The significant accounting policies and estimates used in preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2014, and the notes thereto, which are included in the Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2015.
	Contingent consideration
	Each reporting period, the Company revalues the contingent consideration obligations associated with business combinations to their fair value and record within operating expenses increases in their fair value as contingent consideration expense and decreases in the fair value as contingent consideration income. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s programs in certain indications progress and additional data are obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 4, “Fair value measurements,” for additional information.
	Net income (loss) per share
	Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, employee stock purchase plan, warrants, and acquisition holdback shares using the treasury stock method.
	Use of estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Estimates are used in the following areas, among others: fair value estimates used to assess potential impairment of long-lived assets, contingent consideration, stock-based compensation expense, accrued expenses, revenue and income taxes. Actual results could materially differ from those estimates.

Marketable_Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2015
Investments Debt And Equity Securities [Abstract]
Marketable securities	3. Marketable securities
	The following table summarizes the available-for-sale securities held at March 31, 2015 and December 31, 2014 (in thousands):
	Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
	31-Mar-15
	U.S. government agency securities	$	323,863	$	19	$	(141	)	$	323,741
	Certificates of deposit		12,400		3		(4	)		12,399
	Total	$	336,263	$	22	$	(145	)	$	336,140
	31-Dec-14
	U.S. government agency securities	$	131,589	$	6	$	(59	)	$	131,536
	Certificates of deposit		12,640		—		(18	)		12,622
	Total	$	144,229	$	6	$	(77	)	$	144,158
	No available-for-sale securities held as of March 31, 2015 or December 31, 2014 had remaining maturities greater than two years.

Fair_value_measurements

Fair value measurements	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair value measurements	4. Fair value measurements
	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2015 and December 31, 2014 (in thousands):
	Description		Total		Quoted		Significant		Significant
					prices in		other		unobservable
					active		observable		inputs
					markets		inputs		(Level 3)
					(Level 1)		(Level 2)
	31-Mar-15
	Assets:
	Cash and cash equivalents		$	133,174	$	133,174	$	—	$	—
	Marketable securities:
	U.S. government agency securities			323,741		—		323,741		—
	Certificates of deposit			12,399		—		12,399		—
	Total assets		$	469,314	$	133,174	$	336,140	$	—
	Liabilities:
	Contingent consideration		$	7,011	$	—	$	—	$	7,011
	Total liabilities		$	7,011	$	—	$	—	$	7,011
	31-Dec-14
	Assets:
	Cash and cash equivalents		$	347,845	$	347,845	$	—	$	—
	Marketable securities:
	U.S. government agency securities			131,536		—		131,536		—
	Certificates of deposit			12,622		—		12,622		—
	Total assets		$	492,003	$	347,845	$	144,158	$	—
	Liabilities:
	Contingent consideration		$	6,796	$	—	$	—	$	6,796
	Total liabilities		$	6,796	$	—	$	—	$	6,796
	Cash and cash equivalents
	The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. As of March 31, 2015 and December 31, 2014, cash and cash equivalents comprise funds in cash and money market accounts.
	Marketable securities
	The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At March 31, 2015 and December 31, 2014, the balance in the Company’s accumulated other comprehensive loss was composed solely of activity related to the Company’s available-for-sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the three months ended March 31, 2015, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive income for the same period.
	The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of March 31, 2015 and December 31, 2014 was $248.0 million and $134.4 million, respectively. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of March 31, 2015 and December 31, 2014.
	Contingent consideration
	In connection with the acquisition of Pregenen, the Company recorded contingent consideration pertaining to the amounts potentially payable to Pregenen’s former equityholders pursuant to the Stock Purchase Agreement (the “Stock Purchase Agreement”) by and among the Company, Pregenen and Pregenen’s former equityholders. Contingent consideration is measured at fair value and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the condensed consolidated statements of operations and comprehensive loss.
	Contingent consideration may change significantly as development progresses and additional data are obtained, impacting the Company’s assumptions regarding probabilities of successful achievement of related milestones used to estimate the fair value of the liability and the timing in which they are expected to be achieved. In evaluating the fair value information, considerable judgment is required to interpret the market data used to develop the estimates. The estimates of fair value may not be indicative of the amounts that could be realized in a current market exchange. Accordingly, the use of different market assumptions and/or different valuation techniques could result in materially different fair value estimates.
	The significant unobservable inputs used in the measurement of fair value of the Company’s contingent consideration are probabilities of successful achievement of preclinical, clinical and commercial milestones, the period in which these milestones are expected to be achieved ranging from 2015 to 2026 and discount rates ranging from 10.8% to 14.2%. Significant increases or decreases in any of the probabilities of success would result in a significantly higher or lower fair value measurement, respectively. Significant increases or decreases in these other inputs would result in a significantly lower or higher fair value measurement, respectively.
	The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations which include Level 3 inputs (in thousands):
		Three months ended
		31-Mar-15
	Beginning balance	$	6,796
	Additions		—
	Changes in fair value		215
	Payments		—
	Ending balance	$	7,011
	As of March 31, 2015, $0.3 million of the fair value of the Company’s total contingent consideration obligations was reflected as a component of accrued expenses and other current liabilities within the condensed consolidated balance sheets, with the remaining balance of $6.7 million reflected as a non-current liability.

Commitments_and_contingencies

Commitments and contingencies	3 Months Ended
	Mar. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	5. Commitments and contingencies
	The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at March 31, 2015 and December 31, 2014 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 €1.1 million related to years 2011 through 2013. The Company has applied for €0.7 million related to 2014, which is classified as current assets within the condensed consolidated balance sheets, and the Company has not yet applied for €0.1 million related to the three months ended March 31, 2015, which is classified as non-current assets within the condensed consolidated balance sheets as of March 31, 2015.  The years 2011 through 2015 are open and subject to examination.
	On June 30, 2014, the Company acquired Pregenen. The Company may be required to make up to an additional $135.0 million 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 $15.0 million relates to preclinical milestones, $20.1 million relates to clinical milestones and $99.9 million 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,” for additional information.

Significant_agreements

Significant agreements	3 Months Ended
	Mar. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant agreements	6. Significant agreements
	Celgene Corporation
	On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “Collaboration Agreement”) with Celgene to discover, develop and commercialize potentially disease-altering gene therapies in oncology. The collaboration is focused on applying gene therapy technology to genetically modify a patient’s own T cells, known as chimeric antigen receptor, or CAR, T cells, to target and destroy cancer cells. Additionally, on March 19, 2013, the Company entered into a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with Celgene pursuant to which the Company obtained a sublicense to certain intellectual property from Celgene, originating under Celgene’s license from Baylor College of Medicine, for use in the collaboration.
	Under the terms of the Collaboration Agreement, the Company received a $75.0 million up-front, non-refundable cash payment. The Company is responsible for conducting discovery, research and development activities through completion of Phase I clinical studies, if any, during the initial term of the agreement, or three years. The collaboration is governed by a joint steering committee (“JSC”) formed by an equal number of representatives from the Company and Celgene. The JSC will, among other activities, review the collaboration program, review and evaluate product candidates and approve regulatory plans. In addition to the JSC, the Collaboration Agreement provides that the Company and Celgene each appoint representatives to a patent committee, which is responsible for managing the intellectual property developed and used during the collaboration.
	During each of the three months ended March 31, 2015 and 2014 the Company recognized $6.3 million of revenue associated with its collaboration with Celgene related to the recognition of discovery, research and development services. As of March 31, 2015 and December 31, 2014, there was $24.3 million and $30.7 million of deferred revenue related to the Company’s collaboration with Celgene, which is classified as current or non-current in the condensed consolidated balance sheets based on the contractual term of the arrangement.
	For additional discussion on significant agreements, please refer to Note 11, "Significant agreements," in the 2014 Annual Report on Form 10-K.

Stockbased_compensation_and_wa

Stock-based compensation and warrants	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation and warrants	7. Stock-based compensation and warrants
	In January 2015, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 1.3 million shares as a result of the automatic increase provision of the 2013 Plan. As of March 31, 2015, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 1.0 million.
	Stock-based compensation expense
	Stock-based compensation expense by award type was as follows (in thousands):
		Three months ended
		March 31,
			2015			2014
	Stock options	$	4,755		$	2,477
	Restricted stock awards		—			16
	Restricted stock units		617			—
	Employee stock purchase plan		60			—
		$	5,432		$	2,493
	As of March 31, 2015, there was $78.1 million of unrecognized stock-based compensation expense, net of estimated forfeitures, related to unvested stock options and restricted stock units that is expected to be recognized over a weighted-average period of 2.9 years.
	On January 29, 2015, the Company entered into a Transitional Services and Separation Agreement with its Chief Scientific Officer, ending his employment with the Company effective July 6, 2015. Subsequent to this separation date, he will serve as a member of the Company’s Scientific Advisory Board. Under the terms of the agreement, outstanding options held by the Chief Scientific Officer were modified. The incremental value of the modification was estimated to be $3.0 million using a Black-Scholes option valuation model, which will be recognized within research and development expense on a straight-line basis through the date of separation. As a result of the modification, $1.0 million was recognized during the three months ended March 31, 2015.
	Restricted stock units
	The following table summarizes the restricted stock unit activity under the Company’s equity award plans (share values in thousands):
		Shares			Weighted-average
					grant date
					fair value
	Unvested balance at December 31, 2014		179		$	30.47
	Granted		—			—
	Vested		—			—
	Forfeited		—			—
	Unvested balance at March 31, 2015		179		$	30.47
	Stock options
	The following table summarizes the stock option activity under the Company’s equity award plans (share values in thousands):
		Shares			Weighted-average
					exercise price
					per share
	Outstanding at December 31, 2014		3,652		$	12.3
	Granted		776		$	96.74
	Exercised		(347	)	$	7.07
	Canceled or forfeited		(7	)	$	25.16
	Outstanding at March 31, 2015		4,074		$	28.8
	Exercisable at March 31, 2015		1,297		$	6.91
	Vested and expected to vest at March 31, 2015		3,997		$	29.12
	Options exercisable for approximately 0.3 million shares of common stock were exercised during the three months ended March 31, 2015, resulting in total proceeds to the Company of $2.4 million. In accordance with the stock option plans, the shares were issued from a pool of shares reserved for issuance under the stock option plans described above.
	Employee stock purchase plan
	The Company’s 2013 Employee Stock Purchase Plan (“2013 ESPP”) authorizes the initial issuance of up to a total of 238,000 shares of the Company’s common stock to participating employees. The first offering period under the 2013 ESPP closed on January 31, 2015, resulting in the purchase of 6,780 common shares.
	Warrants
	As of March 31, 2015 and December 31, 2014, the Company had 0.2 million warrants outstanding to purchase common stock. During the three months ended March 31, 2015, there were no warrants exercised and no cancellations or expirations.

Income_taxes

Income taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income taxes	8. Income taxes
	Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. The Company has allocated its valuation allowance in accordance with the provisions of ASC 740, Income Taxes, which resulted in a current deferred tax asset of $1.9 million and a non-current deferred tax liability of $1.9 million as of March 31, 2015.

Net_loss_per_share

Net loss per share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Net loss per share	9. Net loss per share
	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands):
		Three months ended
		March 31,
			2015	2014
	Warrants		177	338
	Outstanding stock options		4,074	4,323
	Unvested restricted stock		—	48
	Restricted stock units		179	—
	ESPP shares		1	—
	Acquisition holdback		94	—
			4,525	4,709

Subsequent_events

Subsequent events	3 Months Ended
	Mar. 31, 2015
Subsequent Events [Abstract]
Subsequent events	bluebird bio, Inc.
	Notes to Condensed Consolidated Financial Statements
	(unaudited)
	10. Subsequent events
	On April 1, 2015, the Company amended an existing license agreement with Institut Pasteur, which resulted in a payment of €3.0 million that was paid during the second quarter of 2015.
	On April 6, 2015, the Company amended and restated an existing patent sublicense agreement, which resulted in a license fee payment of $5.4 million that was paid during the second quarter of 2015.
	On April 10, 2015, the Company modified the vesting conditions of a stock option award held by a non-employee founder, which resulted in $6.7 million of share-based compensation expense that was recognized to research and development expense during the second quarter of 2015.

Summary_of_significant_account1

Summary of significant accounting policies and basis of presentation (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of presentation and principles of consolidation	Basis of presentation and principles of consolidation
	The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended March 31, 2015 and 2014.
	The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2014, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 25, 2015.
	The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Precision Genome Engineering, Inc. (“Pregenen”), bluebird bio France – SARL, bluebird bio Australia Pty Ltd. and bluebird bio Securities Corporation. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. All material long-lived assets of the Company reside in the United States.
Summary of accounting policies	Summary of accounting policies
	The significant accounting policies and estimates used in preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2014, and the notes thereto, which are included in the Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2015.
Contingent consideration	Contingent consideration
	Each reporting period, the Company revalues the contingent consideration obligations associated with business combinations to their fair value and record within operating expenses increases in their fair value as contingent consideration expense and decreases in the fair value as contingent consideration income. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s programs in certain indications progress and additional data are obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 4, “Fair value measurements,” for additional information.
Net income (loss) per share	Net income (loss) per share
	Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, employee stock purchase plan, warrants, and acquisition holdback shares using the treasury stock method.
Use of estimates	Use of estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Estimates are used in the following areas, among others: fair value estimates used to assess potential impairment of long-lived assets, contingent consideration, stock-based compensation expense, accrued expenses, revenue and income taxes. Actual results could materially differ from those estimates.

Marketable_Securities_Tables

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2015
Investments Debt And Equity Securities [Abstract]
Summary of Available for Sale Securities Held	The following table summarizes the available-for-sale securities held at March 31, 2015 and December 31, 2014 (in thousands):
	Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
	31-Mar-15
	U.S. government agency securities	$	323,863	$	19	$	(141	)	$	323,741
	Certificates of deposit		12,400		3		(4	)		12,399
	Total	$	336,263	$	22	$	(145	)	$	336,140
	31-Dec-14
	U.S. government agency securities	$	131,589	$	6	$	(59	)	$	131,536
	Certificates of deposit		12,640		—		(18	)		12,622
	Total	$	144,229	$	6	$	(77	)	$	144,158

Fair_value_measurements_Tables

Fair value measurements (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2015 and December 31, 2014 (in thousands):
	Description		Total		Quoted		Significant		Significant
					prices in		other		unobservable
					active		observable		inputs
					markets		inputs		(Level 3)
					(Level 1)		(Level 2)
	31-Mar-15
	Assets:
	Cash and cash equivalents		$	133,174	$	133,174	$	—	$	—
	Marketable securities:
	U.S. government agency securities			323,741		—		323,741		—
	Certificates of deposit			12,399		—		12,399		—
	Total assets		$	469,314	$	133,174	$	336,140	$	—
	Liabilities:
	Contingent consideration		$	7,011	$	—	$	—	$	7,011
	Total liabilities		$	7,011	$	—	$	—	$	7,011
	31-Dec-14
	Assets:
	Cash and cash equivalents		$	347,845	$	347,845	$	—	$	—
	Marketable securities:
	U.S. government agency securities			131,536		—		131,536		—
	Certificates of deposit			12,622		—		12,622		—
	Total assets		$	492,003	$	347,845	$	144,158	$	—
	Liabilities:
	Contingent consideration		$	6,796	$	—	$	—	$	6,796
	Total liabilities		$	6,796	$	—	$	—	$	6,796
Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations	The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations which include Level 3 inputs (in thousands):
		Three months ended
		31-Mar-15
	Beginning balance	$	6,796
	Additions		—
	Changes in fair value		215
	Payments		—
	Ending balance	$	7,011

Stockbased_compensation_and_wa1

Stock-based compensation and warrants (Tables)	3 Months Ended
	Mar. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Stock-Based Compensation Expense by Award Type	Stock-based compensation expense by award type was as follows (in thousands):
		Three months ended
		March 31,
			2015			2014
	Stock options	$	4,755		$	2,477
	Restricted stock awards		—			16
	Restricted stock units		617			—
	Employee stock purchase plan		60			—
		$	5,432		$	2,493
Summary of Stock Option Activity Under Plan	The following table summarizes the stock option activity under the Company’s equity award plans (share values in thousands):
		Shares			Weighted-average
					exercise price
					per share
	Outstanding at December 31, 2014		3,652		$	12.3
	Granted		776		$	96.74
	Exercised		(347	)	$	7.07
	Canceled or forfeited		(7	)	$	25.16
	Outstanding at March 31, 2015		4,074		$	28.8
	Exercisable at March 31, 2015		1,297		$	6.91
	Vested and expected to vest at March 31, 2015		3,997		$	29.12
Restricted Stock Units R S U [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Restricted Common Stock Awards	Restricted stock units
	The following table summarizes the restricted stock unit activity under the Company’s equity award plans (share values in thousands):
		Shares			Weighted-average
					grant date
					fair value
	Unvested balance at December 31, 2014		179		$	30.47
	Granted		—			—
	Vested		—			—
	Forfeited		—			—
	Unvested balance at March 31, 2015		179		$	30.47

Net_loss_per_share_Tables

Net loss per share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands):
		Three months ended
		March 31,
			2015	2014
	Warrants		177	338
	Outstanding stock options		4,074	4,323
	Unvested restricted stock		—	48
	Restricted stock units		179	—
	ESPP shares		1	—
	Acquisition holdback		94	—
			4,525	4,709

Summary_of_significant_account2

Summary of significant accounting policies and basis of presentation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015
	Segment
Accounting Policies [Abstract]
Number of operating segment	1

Marketable_securities_Summary_

Marketable securities - Summary of Available for Sale Securities Held (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$336,263	$144,229
Unrealized Gains	22	6
Unrealized Losses	-145	-77
Fair Value	336,140	144,158
U.S. Government Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	323,863	131,589
Unrealized Gains	19	6
Unrealized Losses	-141	-59
Fair Value	323,741	131,536
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,400	12,640
Unrealized Gains	3
Unrealized Losses	-4	-18
Fair Value	$12,399	$12,622

Fair_value_measurements_Record

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Assets:
Assets, fair value disclosure, recurring	$469,314	$492,003
Liabilities:
Liabilities, fair value disclosure, recurring	7,011	6,796
Contingent consideration obligations [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	7,011	6,796
Cash and Cash Equivalents [Member]
Assets:
Assets, fair value disclosure, recurring	133,174	347,845
U.S. Government Agency Securities [Member]
Assets:
Assets, fair value disclosure, recurring	323,741	131,536
Certificates of Deposit [Member]
Assets:
Assets, fair value disclosure, recurring	12,399	12,622
Quoted prices in active markets (Level 1) [Member]
Assets:
Assets, fair value disclosure, recurring	133,174	347,845
Quoted prices in active markets (Level 1) [Member] | Cash and Cash Equivalents [Member]
Assets:
Assets, fair value disclosure, recurring	133,174	347,845
Significant other observable inputs (Level 2) [Member]
Assets:
Assets, fair value disclosure, recurring	336,140	144,158
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities [Member]
Assets:
Assets, fair value disclosure, recurring	323,741	131,536
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Assets, fair value disclosure, recurring	12,399	12,622
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	7,011	6,796
Significant unobservable inputs (Level 3) [Member] | Contingent consideration obligations [Member]
Liabilities:
Liabilities, fair value disclosure, recurring	$7,011	$6,796

Fair_value_measurements_Additi

Fair value measurements - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities	Three months or less
Realized gain (loss) on available-for-sale securities	$0
Reclassification out of accumulated other comprehensive income	0
Unrealized Loss on Securities	248,000,000	134,400,000
Investments with other-than-temporary impairment	0	0
Contingent consideration, current	300,000
Contingent consideration, non current	$6,670,000	$6,321,000
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2015
Milestone discount rates	10.80%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2026
Milestone discount rates	14.20%

Fair_value_measurements_RollFo

Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) (Significant unobservable inputs (Level 3) [Member], Contingent consideration obligations [Member], USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015
Significant unobservable inputs (Level 3) [Member] | Contingent consideration obligations [Member]
Fair Value Assets Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	$6,796
Changes in fair value	215
Ending balance	$7,011

Commitments_and_contingencies_

Commitments and contingencies - Additional Information (Detail)	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2013	Dec. 31, 2014
	USD ($)	USD ($)	Pregenen [Member]	Pregenen [Member]	Pregenen [Member]	Pregenen [Member]	French Credit d'Impot Recherche Program [Member]	French Credit d'Impot Recherche Program [Member]	French Credit d'Impot Recherche Program [Member]
			USD ($)	Preclinical milestones [Member]	Clinical Milestone Payments [Member]	Commercial Milestones Payments [Member]	EUR (€)	EUR (€)	EUR (€)
				USD ($)	USD ($)	USD ($)
Commitments And Contingencies Disclosure [Line Items]
Eligible percentage of research expense monetized							30.00%
Aggregate reimbursement received related to years 2011 through 2013								€ 1,100,000
Current assets									700,000
Other non-current assets	1,294,000	1,215,000					100,000
Contingent cash payments			$135,000,000	$15,000,000	$20,100,000	$99,900,000

Significant_agreements_Additio

Significant agreements - Additional Information (Detail) (Celgene Corporation [Member], USD $)	0 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Mar. 19, 2013	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Term of collaboration agreement	3 years
Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue recognized		$6.30	$6.30
Deferred revenue		24.3		30.7
Up-front Payment Arrangement [Member] | Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received	75

Stockbased_compensation_and_wa2

Stock-based compensation and warrants - Additional Information (Detail) (USD $)	0 Months Ended	1 Months Ended	3 Months Ended
In Millions, except Share data, unless otherwise specified	Jan. 29, 2015	Jan. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan		1,300,000
Number of shares available for issuance			1,000,000
Incremental value on option valuation	$3
Stock option share exercised			347,000
Proceed from option share exercised			2.4
Warrants outstanding			200,000	200,000
Warrants exercised			0
Cancellation and expiration of warrants			0
Research and Development Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Incremental value on option valuation			1
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options and restricted stock awards			78.1
Expected period for stock option			2 years 10 months 24 days
Restricted Stock Units R S U [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options and restricted stock awards			$78.10
Expected period for stock option			2 years 10 months 24 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common shares reserved for future issuance			238,000
Shares purchased under the plan		6,780

Stockbased_compensation_and_wa3

Stock-based compensation and warrants - Summary of Stock-Based Compensation Expense by Award Type (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$5,432	$2,493
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	4,755	2,477
Restricted Stock Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		16
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	617
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$60

Stockbased_compensation_and_wa4

Stock-based compensation and warrants - Summary of Restricted Stock Units(Detail) (Restricted Stock Units R S U [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, except Per Share data, unless otherwise specified
Restricted Stock Units R S U [Member]
Shares
Unvested balance at beginning of period	179	179
Unvested balance at end of period	179	179
Weighted-average grant date fair value
Unvested balance at beginning of period	$30.47	$30.47
Unvested balance at end of period	$30.47	$30.47

Stockbased_compensation_and_wa5

Stock-based compensation and warrants - Summary of Stock Option Activity Under Plan (Detail) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015
Shares
Outstanding at beginning of period	3,652
Granted	776
Exercised	-347
Canceled or forfeited	-7
Outstanding at end of period	4,074
Exercisable at end of period	1,297
Vested and expected to vest at end of period	3,997
Weighted-average exercise price per share
Outstanding at beginning of period	$12.30
Granted	$96.74
Exercised	$7.07
Canceled or forfeited	$25.16
Outstanding at end of period	$28.80
Exercisable at end of period	$6.91
Vested and expected to vest at end of period	$29.12

Income_taxes_Additional_Inform

Income taxes - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Income Tax Disclosure [Abstract]
Current deferred tax asset	$1,913	$1,913
Non-current deferred tax liability	$1,913	$1,913

Net_loss_per_share_Common_Stoc

Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	4,525	4,709
Warrants [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	177	338
Outstanding stock options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	4,074	4,323
Restricted Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share		48
Restricted Stock Units R S U [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	179
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	1
Acquisition holdback [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	94

Subsequent_events_Additional_I

Subsequent events - Additional Information (Detail)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 10, 2015	Apr. 06, 2015	Apr. 01, 2015
	USD ($)	USD ($)	Subsequent Event [Member]	Subsequent Event [Member]	Subsequent Event [Member]
			Research and Development Expense [Member]	Sublicense Agreement [Member]	Pasteur [Member]
			USD ($)	USD ($)	EUR (€)
Subsequent Event [Line Items]
License fee payment				$5,400,000	€ 3,000,000
Stock-based compensation expense	$5,432,000	$2,493,000	$6,700,000