Significant Agreements - Other Agreements (Details) $ in Thousands	1 Months Ended							12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 shares	Aug. 31, 2017 shares	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($) item	Oct. 31, 2014 USD ($) shares	Aug. 31, 2014 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2017 USD ($)	Dec. 16, 2016 USD ($)
Notes payable	$ 7,500			$ 10,000				$ 7,500	$ 10,000
Research and development								83,159	$ 56,979	$ 18,846
The General Hospital (MGH) | License Patent Rights
Upfront fee							$ 100
Stock issued to licensors | shares							66,848
Percentage of outstanding shares issued							0.50%
Research and development					$ 800
Market capitalization threshold					1,000,000
Market capitalization	1,000,000							$ 1,000,000
The General Hospital (MGH) | Maximum | License Patent Rights
Potential liability for future clinical, regulatory and commercial milestone payments related to product approval					1,000		$ 3,700
Potential liability for future sales milestone payments							$ 1,800
Potential liability for future clinical and regulatory milestone payments related to product approval					$ 4,900
Percentage of net sales threshold for potential milestone payments to be made					1.00%
Number of licensed products | item					4
Success Payments					$ 6,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | License Patent Rights
Upfront fee						$ 200
Stock issued to licensors | shares						561,531
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Cpf1 License Agreement
License Agreement Contract Termination Term								4 months
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Cpf1 Market Cap Success Payments
Market capitalization	1,000,000							$ 1,000,000			$ 750,000
Notes payable payment terms								150 days
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Cas9-II License Agreement
Market capitalization	1,000,000							$ 1,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Maximum | Cpf1 License Agreement
Research and development				$ 16,500
Royalties credit paid to third party (as a percent)								50.00%
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Maximum | Cpf1 Success Payments
Success Payments								$ 10,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Maximum | Cpf1 Market Cap Success Payments
Success Payments								125,000
Market capitalization threshold	10,000,000							10,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Maximum | Cas9-II License Agreement
Success Payments								30,000
Market capitalization threshold	9,000,000							9,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Minimum | Cpf1 Success Payments
Success Payments								750,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Minimum | Cas9-II License Agreement
Market capitalization threshold	1,000,000							1,000,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Rare disease | Maximum
Potential liability for future sales milestone payments						$ 36,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | Rare disease | Maximum | Cpf1 License Agreement
Potential liability for future sales milestone payments	36,000							36,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | United States Europe And Japan | Maximum | License Patent Rights
Potential liability for future clinical and regulatory milestone payments related to product approval						14,800
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | United States Europe And Japan | Human disease | Maximum | Cpf1 License Agreement
Potential liability for future clinical and regulatory milestone payments related to product approval	20,000							20,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | United States Europe And Japan | Human disease | Maximum | Cas9-II License Agreement
Potential liability for future sales milestone payments	13,500							13,500
Potential liability for future clinical and regulatory milestone payments related to product approval												$ 3,700
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | United States Europe And Japan | Rare disease | Maximum | Cpf1 License Agreement
Potential liability for future clinical and regulatory milestone payments related to product approval	6,000							6,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | United States Europe And Japan | Rare disease | Maximum | Cas9-II License Agreement
Potential liability for future sales milestone payments	9,000							9,000
Potential liability for future clinical and regulatory milestone payments related to product approval	1,100							1,100
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | United States and one other | Human disease | Maximum | License Patent Rights
Potential liability for future clinical and regulatory milestone payments related to product approval						4,100
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | UNITED STATES | Human disease | Maximum | License Patent Rights
Potential liability for future sales milestone payments						$ 54,000
Broad, Massachusetts Institute of Technology, or MIT, and Harvard (Institutions) | UNITED STATES | Human disease | Maximum | Cpf1 License Agreement
Potential liability for future sales milestone payments	54,000							$ 54,000
The Broad Institute Inc | Cpf1 License Agreement
Stock issued to licensors | shares		371,166	108,104
Success Payments	$ 7,500