Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 25,800	$ 57,763	$ 21,553	$ 13,076	$ 54,851	$ 21,958	$ 17,548	$ 8,071	$ 118,192	$ 102,428	$ 84,452
Operating expenses	69,232	61,464	59,450	57,598	53,382	51,206	51,052	51,968	247,744	207,608	161,602
Net loss	(40,675)	(1,508)	(35,965)	(42,974)	4,501	(27,361)	(30,356)	(42,203)	(121,122)	(95,419)	(68,889)
Net loss attributable to non-controlling interest	(71)	42	(36)	(61)	(54)	(54)	(72)	(53)	(126)	(233)	(555)
Net (loss) income attributable to Sangamo Therapeutics, Inc.	$ (40,604)	$ (1,550)	$ (35,929)	$ (42,913)	$ 4,555	$ (27,307)	$ (30,284)	$ (42,150)	$ (120,996)	$ (95,186)	$ (68,334)
Net loss per share - basic and diluted (in dollars per share)	$ (0.29)	$ (0.01)	$ (0.26)	$ (0.37)	$ 0.04	$ (0.24)	$ (0.26)	$ (0.41)	$ (0.90)	$ (0.85)	$ (0.70)