Summary of significant accounting policies and basis of presentation (Policies)	9 Months Ended
Sep. 30, 2014
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 September 30, 2014 and 2013. 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, 2013, 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 March 5, 2014. The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Precision Genome Engineering, Inc., bluebird bio France – SARL, bluebird bio Australia Pty Ltd. and bluebird bio Securities Corporation. All intercompany balances and transactions have been eliminated in consolidation. The assets acquired and liabilities assumed in connection with the Company’s acquisition of Pregenen were recorded at their fair values as of June 30, 2014, the date of the acquisition, and the operating results of Pregenen have been consolidated with those of the Company from the date of acquisition. 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 described in the Company’s audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, which are included in the Annual Report on Form 10-K, have had no material changes during the nine months ended September 30, 2014, except as noted below:
Marketable securities	Marketable securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current. Available-for-sale securities are maintained by an investment manager and may consist of U.S. Treasury securities, U.S. government agency securities, certificates of deposit and money market funds invested in U.S. Treasuries or U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, mark the investment to market through a charge to the Company’s statement of operations and comprehensive loss.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and available-for-sale securities. The Company maintains its cash and cash equivalent balances with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s available-for-sale investments primarily consist of U.S. government agency securities and certificates of deposit and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy which limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to be at least AA+/Aa1 rated, thereby reducing credit risk exposure.
Business combinations	Business combinations On June 30, 2014, the Company completed its acquisition of Pregenen for total consideration of $30,991, consisting of cash consideration of $5,093, common stock consideration of $19,348 and contingent consideration with an estimated fair value of $6,550 on the date of purchase. The estimated fair value of the contingent consideration is based upon significant assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and discount rates. The estimated fair value could materially differ from actual values or fair values determined using different assumptions. See Note 4, “Fair value measurements,” for additional information. This transaction was accounted for as a business combination under the acquisition method of accounting. Accordingly, the tangible assets and identifiable intangible assets acquired and liabilities assumed were recorded at fair value as of the date of acquisition, with the remaining purchase price recorded as goodwill. The estimated fair values of the assets acquired and liabilities assumed at the date of acquisition are summarized in Note 7, “Business combinations.” The estimated fair values of acquired assets and assumed liabilities were determined using the methods discussed in the following paragraphs and require significant judgment and estimates, which could materially differ from actual values and fair values determined using different methods or assumptions.
Goodwill	Goodwill Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company has not recognized any impairment charges related to goodwill.
Intangible assets	Intangible assets Intangible assets consist of acquired core technology with finite lives. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives. The Company evaluates the potential impairment of intangible assets if events or changes in circumstances indicate that the carrying amount of the assets may not be fully recoverable or that the useful lives of these assets are no longer appropriate. The impairment test is based on a comparison of the undiscounted cash flows expected to be generated from the use of the asset group and its eventual disposition to the carrying value of the asset group. If impairment is indicated, the asset is written down by the amount by which the carrying value of the asset exceeds the related fair value of the asset. The Company has not recognized an impairment charge related to intangible assets.
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.
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. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates are used in the following areas, among others: acquisition-date fair value and subsequent fair value estimates used to assess potential impairment of long-lived assets, including goodwill and intangible assets, contingent consideration, stock-based compensation expense, accrued expenses, revenue and income taxes.
Fair value of financial instruments	Fair value of financial instruments The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. ASC 820, Fair Value Measurements and Disclosures, establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below: Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term maturities.
Other Income and Expense	Other Income and Expense In March 2014, the Company received an award of $306 of tax incentives from the Massachusetts Life Sciences Center, which will allow the Company to monetize approximately $276 of state research and development tax credits. In exchange for these incentives, the Company pledged to hire an incremental fifteen employees and to maintain the additional headcount through at least December 31, 2018. Failure to do so could result in the Company being required to repay some or all of these incentives. As the Company met the additional headcount condition during the quarter ended June 30, 2014 and continues to meet such condition during the quarter ended September 30, 2014, the Company continues to defer and amortize the benefit of this monetization on a straight-line basis over the five-year performance period in other income (expense).
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.
Recent accounting pronouncements	Recent accounting pronouncements During the quarter ended June 30, 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU No. 2014-09”), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective on January 1, 2017 and early adoption is not permitted for public entities. The Company is currently evaluating the potential impact that ASU No. 2014-09 may have on its financial position and results of operations.

Summary of significant accounting policies and basis of presentation (Policies)

9 Months Ended

Sep. 30, 2014

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 September 30, 2014 and 2013.

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, 2013, 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 March 5, 2014.

The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Precision Genome Engineering, Inc., bluebird bio France – SARL, bluebird bio Australia Pty Ltd. and bluebird bio Securities Corporation. All intercompany balances and transactions have been eliminated in consolidation. The assets acquired and liabilities assumed in connection with the Company’s acquisition of Pregenen were recorded at their fair values as of June 30, 2014, the date of the acquisition, and the operating results of Pregenen have been consolidated with those of the Company from the date of acquisition. 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 described in the Company’s audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, which are included in the Annual Report on Form 10-K, have had no material changes during the nine months ended September 30, 2014, except as noted below:

Marketable securities

The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current. Available-for-sale securities are maintained by an investment manager and may consist of U.S. Treasury securities, U.S. government agency securities, certificates of deposit and money market funds invested in U.S. Treasuries or U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense. Realized gains and losses are determined using the specific identification method and are included in other income (expense).

If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, mark the investment to market through a charge to the Company’s statement of operations and comprehensive loss.

Concentrations of Credit Risk

Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and available-for-sale securities. The Company maintains its cash and cash equivalent balances with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s available-for-sale investments primarily consist of U.S. government agency securities and certificates of deposit and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy which limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to be at least AA+/Aa1 rated, thereby reducing credit risk exposure.

Business combinations

On June 30, 2014, the Company completed its acquisition of Pregenen for total consideration of $30,991, consisting of cash consideration of $5,093, common stock consideration of $19,348 and contingent consideration with an estimated fair value of $6,550 on the date of purchase. The estimated fair value of the contingent consideration is based upon significant assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and discount rates. The estimated fair value could materially differ from actual values or fair values determined using different assumptions. See Note 4, “Fair value measurements,” for additional information.

This transaction was accounted for as a business combination under the acquisition method of accounting. Accordingly, the tangible assets and identifiable intangible assets acquired and liabilities assumed were recorded at fair value as of the date of acquisition, with the remaining purchase price recorded as goodwill. The estimated fair values of the assets acquired and liabilities assumed at the date of acquisition are summarized in Note 7, “Business combinations.” The estimated fair values of acquired assets and assumed liabilities were determined using the methods discussed in the following paragraphs and require significant judgment and estimates, which could materially differ from actual values and fair values determined using different methods or assumptions.

Goodwill

Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company has not recognized any impairment charges related to goodwill.

Intangible assets

Intangible assets consist of acquired core technology with finite lives. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives. The Company evaluates the potential impairment of intangible assets if events or changes in circumstances indicate that the carrying amount of the assets may not be fully recoverable or that the useful lives of these assets are no longer appropriate. The impairment test is based on a comparison of the undiscounted cash flows expected to be generated from the use of the asset group and its eventual disposition to the carrying value of the asset group. If impairment is indicated, the asset is written down by the amount by which the carrying value of the asset exceeds the related fair value of the asset. The Company has not recognized an impairment charge related to intangible assets.

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.

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. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates are used in the following areas, among others: acquisition-date fair value and subsequent fair value estimates used to assess potential impairment of long-lived assets, including goodwill and intangible assets, contingent consideration, stock-based compensation expense, accrued expenses, revenue and income taxes.

Fair value of financial instruments

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. ASC 820, Fair Value Measurements and Disclosures, establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below:

Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly.

Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term maturities.

Other Income and Expense

In March 2014, the Company received an award of $306 of tax incentives from the Massachusetts Life Sciences Center, which will allow the Company to monetize approximately $276 of state research and development tax credits. In exchange for these incentives, the Company pledged to hire an incremental fifteen employees and to maintain the additional headcount through at least December 31, 2018. Failure to do so could result in the Company being required to repay some or all of these incentives. As the Company met the additional headcount condition during the quarter ended June 30, 2014 and continues to meet such condition during the quarter ended September 30, 2014, the Company continues to defer and amortize the benefit of this monetization on a straight-line basis over the five-year performance period in other income (expense).

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.

Recent accounting pronouncements

During the quarter ended June 30, 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU No. 2014-09”), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective on January 1, 2017 and early adoption is not permitted for public entities. The Company is currently evaluating the potential impact that ASU No. 2014-09 may have on its financial position and results of operations.