Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 54,851	$ 21,958	$ 17,548	$ 8,071	$ 26,837	$ 23,562	$ 21,416	$ 12,637	$ 102,428	$ 84,452	$ 36,567
Operating expenses	53,382	51,206	51,052	51,968	47,609	39,803	40,556	33,634	207,608	161,602	92,928
Net loss	4,501	(27,361)	(30,356)	(42,203)	(19,219)	(12,843)	(16,640)	(20,187)	(95,419)	(68,889)	(54,568)
Net loss attributable to non-controlling interest	(54)	(54)	(72)	(53)	(555)	0	0	0	(233)	(555)	0
Net (loss) income attributable to Sangamo Therapeutics, Inc.	$ 4,555	$ (27,307)	$ (30,284)	$ (42,150)	$ (18,664)	$ (12,843)	$ (16,640)	$ (20,187)	$ (95,186)	$ (68,334)	$ (54,568)
Basic and diluted net (loss) income per share attributable to Sangamo Therapeutics, Inc. (in dollars per share)	$ 0.04	$ (0.24)	$ (0.26)	$ (0.41)	$ (0.18)	$ (0.13)	$ (0.17)	$ (0.23)	$ (0.85)	$ (0.70)	$ (0.70)