The Company and Basis of Presentation	9 Months Ended
Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
The Company and Basis of Presentation	1. The Company and Basis of Presentation Organization and Description of Business Sierra Oncology, Inc. (together with its subsidiaries, collectively referred to as the “Company”), a Delaware corporation, is a clinical stage drug development company advancing next generation DNA Damage Response (DDR) therapeutics for the treatment of patients with cancer. Sierra Oncology’s lead drug candidate is SRA737, a potent, highly selective, orally bioavailable small molecule inhibitor of Checkpoint kinase 1 (Chk1), a key regulator of important cell cycle checkpoints and central mediator of the DDR network. Sierra Oncology is also advancing SRA141, a potent, selective and orally bioavailable small molecule inhibitor of cell division cycle 7 kinase (Cdc7) undergoing preclinical development. The Company’s primary activities since inception have been conducting research and development activities, conducting preclinical and clinical testing, recruiting personnel, performing business and financial planning, identifying and evaluating additional drug candidates for potential in-licensing or acquisition, and raising capital to support development activities. The Company has not generated any product revenue related to its primary business purpose to date, nor has it generated any income, and is subject to a number of risks and uncertainties, which include dependence on key individuals, the need to identify and successfully develop commercially viable products, the need to obtain regulatory approval for its products and commercialize them, and the need to obtain adequate additional financing to fund the development of its product candidates. Follow-On Offering On February 14, 2017, the Company completed an underwritten public offering of 19,500,000 shares of common stock. As part of the underwritten public offering, on February 21, 2017 the Company issued an additional 2,347,636 shares of common stock representing the underwriters’ exercise of a majority of their over-allotment option. All shares were offered by the Company at a price to the public of $1.35 per share. The aggregate net proceeds received by the Company from the offering were $27.4 million, net of underwriting discounts and commissions and offering expenses of $2.1 million.

1. The Company and Basis of Presentation

Organization and Description of Business

Sierra Oncology, Inc. (together with its subsidiaries, collectively referred to as the “Company”), a Delaware corporation, is a clinical stage drug development company advancing next generation DNA Damage Response (DDR) therapeutics for the treatment of patients with cancer. Sierra Oncology’s lead drug candidate is SRA737, a potent, highly selective, orally bioavailable small molecule inhibitor of Checkpoint kinase 1 (Chk1), a key regulator of important cell cycle checkpoints and central mediator of the DDR network. Sierra Oncology is also advancing SRA141, a potent, selective and orally bioavailable small molecule inhibitor of cell division cycle 7 kinase (Cdc7) undergoing preclinical development.

The Company’s primary activities since inception have been conducting research and development activities, conducting preclinical and clinical testing, recruiting personnel, performing business and financial planning, identifying and evaluating additional drug candidates for potential in-licensing or acquisition, and raising capital to support development activities.

The Company has not generated any product revenue related to its primary business purpose to date, nor has it generated any income, and is subject to a number of risks and uncertainties, which include dependence on key individuals, the need to identify and successfully develop commercially viable products, the need to obtain regulatory approval for its products and commercialize them, and the need to obtain adequate additional financing to fund the development of its product candidates.

Follow-On Offering

On February 14, 2017, the Company completed an underwritten public offering of 19,500,000 shares of common stock. As part of the underwritten public offering, on February 21, 2017 the Company issued an additional 2,347,636 shares of common stock representing the underwriters’ exercise of a majority of their over-allotment option. All shares were offered by the Company at a price to the public of $1.35 per share. The aggregate net proceeds received by the Company from the offering were $27.4 million, net of underwriting discounts and commissions and offering expenses of $2.1 million.