Nature of Operations	9 Months Ended
Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Operations	1. Nature of Operations Sage Therapeutics, Inc. (“Sage” or the “Company”) is a clinical-stage biopharmaceutical company committed to developing and commercializing novel medicines to treat life-altering central nervous system, or CNS, disorders, where there are no approved therapies or existing therapies are inadequate. The Company has a portfolio of product candidates with a current focus on modulating two critical CNS receptor systems, GABA and NMDA. The GABA receptor family, which is recognized as the major inhibitory neurotransmitter in the CNS, mediates downstream neurologic and bodily function via activation of GABA_A receptors. The NMDA-type receptors of the glutamate receptor system are a major excitatory receptor system in the CNS. Dysfunction in these systems is implicated in a broad range of CNS disorders. The Company is targeting CNS indications where patient populations are easily identified, clinical endpoints are well-defined, and development pathways are feasible. The Company was incorporated under the laws of the State of Delaware on April 16, 2010, and commenced operations on January 19, 2011 as Sterogen Biopharma, Inc. On September 13, 2011, the Company changed its name to Sage Therapeutics, Inc. The Company is subject to risks and uncertainties common to companies in the biotech industry, including, but not limited to, the risks associated with developing product candidates at each stage of non-clinical and clinical development; the challenges associated with gaining regulatory approval of such product candidates; the risks associated with commercializing pharmaceutical products, if approved for marketing and sale; the potential for development by third parties of new technological innovations that may compete with the Company’s products; the dependence on key personnel; the challenges of protecting proprietary technology; the need to comply with government regulations; the high costs of drug development; and the uncertainty of being able to secure additional capital when needed to fund operations. The Company has incurred losses and negative cash flows from operations since its inception. As of September 30, 2017, the Company had an accumulated deficit of $521.0 million. From its inception through September 30, 2017, the Company received net proceeds of $643.3 million from the sales of redeemable convertible preferred stock, the issuance of convertible notes, and the proceeds from its initial public offering (“IPO”) in July 2014 and follow-on underwritten public offerings in April 2015, January 2016 and September 2016. Until such time, if ever, as the Company can generate substantial product revenue, the Company expects to finance its cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances, licensing arrangements and other sources of funding. If the Company is unable to raise additional funds through equity or debt financings when needed, the Company may be required to delay, limit, reduce or terminate product development or future commercialization efforts or grant rights to develop and market products or product candidates that the Company would otherwise prefer to develop and market itself. Based on its current operating plans, the Company believes its cash, cash equivalents and marketable securities of $243.5 million as of September 30, 2017 will be sufficient to fund its anticipated level of operations and capital expenditures into the second quarter of 2018. Under Accounting Standards Update, or ASU, 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40), or ASC 205-40, the Company has the responsibility to evaluate whether conditions and/or events raise substantial doubt about its ability to meet its future financial obligations as they become due within one year after the date that the financial statements are issued. As required by ASC 205-40, this evaluation shall initially not take into consideration the potential mitigating effects of plans that have not been fully implemented as of the date the financial statements are issued. The Company anticipates that its current capital resources will enable it to meet its anticipated operational expenses and capital expenditures into the second quarter of calendar year 2018. As of September 30, 2017, management has assessed the Company’s ability to continue as a going concern in accordance with the requirements of ASC 205-40. Management has determined that the Company’s accumulated deficit, history of losses, and future expected losses meet the ASC 205-40 standard for raising substantial doubt about the Company’s ability to continue as a going concern within one year of the issuance date of these condensed consolidated financial statements. As of September 30, 2017, the Company had an accumulated deficit of $521.0 million and cash, cash equivalents and short-term investments on hand of $243.5 million. The Company is currently forecasting a significant increase in expenditures to support a potential filing of a New Drug Application, or NDA, for brexanolone in postpartum depression, or PPD, in the U.S. and future potential regulatory filings in the European Union, or EU, if the results of the Phase 3 clinical trials in PPD are positive and support such filings, continued preparations for a potential future commercial launch of brexanolone, if successfully developed and approved, and spending on other clinical programs in development. The Company’s current financial resources would not be sufficient to fund the Company’s currently forecasted operating plan for a one year period from the issuance of these condensed consolidated financial statements. The Company has developed plans to mitigate this risk, which primarily consist of raising additional capital through a combination of equity or debt financings, and, depending on the availability and level of additional financings, potentially new collaborations and reducing cash expenditures. The Company currently expects to seek additional funding. While the Company has raised capital in the past, the ability to raise capital in future periods is not considered probable, as defined under the accounting standards. As such, under the requirements of ASC 205-40, management may not consider the potential for future capital raises in their assessment of the Company’s ability to meet its obligations for the next 12 months. If the Company is not able to secure adequate additional funding, the Company plans to make reductions in spending. In that event, the Company may have to delay, reduce the scope of, suspend or eliminate one or more research and development programs or reduce spending and hiring in connection with the Company’s commercialization efforts. The actions necessary to reduce spending under this plan at a level that mitigates the factors described above is not considered probable, as defined in the accounting standards; as such, under the requirements of ASC 205-40, the full extent to which management may extend the Company’s funds through these actions may not be considered in management’s assessment of the Company’s ability to continue as a going concern for the next 12 months as defined by ASC 205-40. As a result, in accordance with the requirements of ASC 205-40, management has concluded that it is required to disclose that substantial doubt exists about the Company’s ability to continue as a going concern for one year from the date these financial statements are issued. While management has plans in place to mitigate these actions, they are not considered probable, as defined in the accounting standards, and a failure to raise the additional funding or to effectively implement cost reductions could harm the Company’s business, results of operations and future prospects. The accompanying unaudited interim condensed consolidated financial statements of the Company have been prepared on a going-concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. The accompanying unaudited interim condensed consolidated financial statements do not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classification of liabilities that may result from uncertainty related to the ability to continue as a going concern.

1. Nature of Operations

Sage Therapeutics, Inc. (“Sage” or the “Company”) is a clinical-stage biopharmaceutical company committed to developing and commercializing novel medicines to treat life-altering central nervous system, or CNS, disorders, where there are no approved therapies or existing therapies are inadequate. The Company has a portfolio of product candidates with a current focus on modulating two critical CNS receptor systems, GABA and NMDA. The GABA receptor family, which is recognized as the major inhibitory neurotransmitter in the CNS, mediates downstream neurologic and bodily function via activation of GABA_A receptors. The NMDA-type receptors of the glutamate receptor system are a major excitatory receptor system in the CNS. Dysfunction in these systems is implicated in a broad range of CNS disorders. The Company is targeting CNS indications where patient populations are easily identified, clinical endpoints are well-defined, and development pathways are feasible.

The Company was incorporated under the laws of the State of Delaware on April 16, 2010, and commenced operations on January 19, 2011 as Sterogen Biopharma, Inc. On September 13, 2011, the Company changed its name to Sage Therapeutics, Inc.

The Company is subject to risks and uncertainties common to companies in the biotech industry, including, but not limited to, the risks associated with developing product candidates at each stage of non-clinical and clinical development; the challenges associated with gaining regulatory approval of such product candidates; the risks associated with commercializing pharmaceutical products, if approved for marketing and sale; the potential for development by third parties of new technological innovations that may compete with the Company’s products; the dependence on key personnel; the challenges of protecting proprietary technology; the need to comply with government regulations; the high costs of drug development; and the uncertainty of being able to secure additional capital when needed to fund operations.

The Company has incurred losses and negative cash flows from operations since its inception. As of September 30, 2017, the Company had an accumulated deficit of $521.0 million. From its inception through September 30, 2017, the Company received net proceeds of $643.3 million from the sales of redeemable convertible preferred stock, the issuance of convertible notes, and the proceeds from its initial public offering (“IPO”) in July 2014 and follow-on underwritten public offerings in April 2015, January 2016 and September 2016. Until such time, if ever, as the Company can generate substantial product revenue, the Company expects to finance its cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances, licensing arrangements and other sources of funding. If the Company is unable to raise additional funds through equity or debt financings when needed, the Company may be required to delay, limit, reduce or terminate product development or future commercialization efforts or grant rights to develop and market products or product candidates that the Company would otherwise prefer to develop and market itself.

Based on its current operating plans, the Company believes its cash, cash equivalents and marketable securities of $243.5 million as of September 30, 2017 will be sufficient to fund its anticipated level of operations and capital expenditures into the second quarter of 2018.

Under Accounting Standards Update, or ASU, 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40), or ASC 205-40, the Company has the responsibility to evaluate whether conditions and/or events raise substantial doubt about its ability to meet its future financial obligations as they become due within one year after the date that the financial statements are issued. As required by ASC 205-40, this evaluation shall initially not take into consideration the potential mitigating effects of plans that have not been fully implemented as of the date the financial statements are issued.

The Company anticipates that its current capital resources will enable it to meet its anticipated operational expenses and capital expenditures into the second quarter of calendar year 2018. As of September 30, 2017, management has assessed the Company’s ability to continue as a going concern in accordance with the requirements of ASC 205-40. Management has determined that the Company’s accumulated deficit, history of losses, and future expected losses meet the ASC 205-40 standard for raising substantial doubt about the Company’s ability to continue as a going concern within one year of the issuance date of these condensed consolidated financial statements. As of September 30, 2017, the Company had an accumulated deficit of $521.0 million and cash, cash equivalents and short-term investments on hand of $243.5 million. The Company is currently forecasting a significant increase in expenditures to support a potential filing of a New Drug Application, or NDA, for brexanolone in postpartum depression, or PPD, in the U.S. and future potential regulatory filings in the European Union, or EU, if the results of the Phase 3 clinical trials in PPD are positive and support such filings, continued preparations for a potential future commercial launch of brexanolone, if successfully developed and approved, and spending on other clinical programs in development. The Company’s current financial resources would not be sufficient to fund the Company’s currently forecasted operating plan for a one year period from the issuance of these condensed consolidated financial statements. The Company has developed plans to mitigate this risk, which primarily consist of raising additional capital through a combination of equity or debt financings, and, depending on the availability and level of additional financings, potentially new collaborations and reducing cash expenditures.

The Company currently expects to seek additional funding. While the Company has raised capital in the past, the ability to raise capital in future periods is not considered probable, as defined under the accounting standards. As such, under the requirements of ASC 205-40, management may not consider the potential for future capital raises in their assessment of the Company’s ability to meet its obligations for the next 12 months.

If the Company is not able to secure adequate additional funding, the Company plans to make reductions in spending. In that event, the Company may have to delay, reduce the scope of, suspend or eliminate one or more research and development programs or reduce spending and hiring in connection with the Company’s commercialization efforts. The actions necessary to reduce spending under this plan at a level that mitigates the factors described above is not considered probable, as defined in the accounting standards; as such, under the requirements of ASC 205-40, the full extent to which management may extend the Company’s funds through these actions may not be considered in management’s assessment of the Company’s ability to continue as a going concern for the next 12 months as defined by ASC 205-40.

As a result, in accordance with the requirements of ASC 205-40, management has concluded that it is required to disclose that substantial doubt exists about the Company’s ability to continue as a going concern for one year from the date these financial statements are issued. While management has plans in place to mitigate these actions, they are not considered probable, as defined in the accounting standards, and a failure to raise the additional funding or to effectively implement cost reductions could harm the Company’s business, results of operations and future prospects.

The accompanying unaudited interim condensed consolidated financial statements of the Company have been prepared on a going-concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. The accompanying unaudited interim condensed consolidated financial statements do not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classification of liabilities that may result from uncertainty related to the ability to continue as a going concern.