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Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Jan. 31, 2018	Sep. 30, 2017	Aug. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Gross proceeds from issuance of shares				$ 154,143	$ 97,488
Cpf1 License Agreement \| The Broad Institute Inc
Subsequent Event [Line Items]
Common stock issued		371,166	108,104
Subsequent Event
Subsequent Event [Line Items]
Payments for purchase of assets	$ 1,600
Number of shares issued under asset purchase agreement	56,099
Price per share	$ 29.76
Subsequent Event \| 2016 License Agreements \| The General Hospital (MGH)
Subsequent Event [Line Items]
Common stock issued	80,000
Subsequent Event \| March 2017 ATM program \| Cowen and Company LLC (“Cowen)
Subsequent Event [Line Items]
Number of common stock issued	1,429,205
Price per share	$ 34.99
Gross proceeds from issuance of shares	$ 50,000
Success Payment Notes \| Cpf1 License Agreement \| The Broad Institute Inc
Subsequent Event [Line Items]
Common stock issued			271,347
Success Payment Notes \| Subsequent Event \| Cpf1 License Agreement \| The Broad Institute Inc
Subsequent Event [Line Items]
Common stock issued	225,909