Unaudited Quarterly Financial Information (Details) - 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
Operating revenue	$ 125,838	$ 113,997	$ 102,965	$ 77,682	$ 98,718	$ 124,853	$ 99,086	$ 75,042	$ 420,482	$ 397,699	$ 389,225
Operating Income (Loss)	6,016	17,084	21,971	12,408	17,482	25,828	22,799	7,331	57,479	73,440	101,851
Net income	(5,486)	9,478	13,538	5,873	8,823	15,788	12,871	1,285	23,403	38,767	59,204
Comprehensive income	$ (5,360)	$ 9,478	$ 13,538	$ 5,873	$ 8,823	$ 15,788	$ 12,871	$ 1,285	$ 23,529	$ 38,767	$ 59,883
SJW Group earnings per share
—Basic (in dollars per share)	$ (0.20)	$ 0.33	$ 0.48	$ 0.21	$ 0.38	$ 0.77	$ 0.63	$ 0.06	$ 0.82	$ 1.83	$ 2.89
—Diluted (in dollars per share)	(0.19)	0.33	0.47	0.21	0.38	0.76	0.62	0.06	$ 0.82	$ 1.82	$ 2.86
Dividends per share	0.30	0.30	0.30	0.30	0.28	0.28	0.28	0.28
--- High (in dollars per share) [Member]
SJW Group earnings per share
Market Price, Stock	74.47	69.23	63.40	63.76	65.31	67.29	68.15	63.47
--- Low (in dollars per share) [Member]
SJW Group earnings per share
Market Price, Stock	$ 67.13	$ 61.23	$ 59.83	$ 54.74	$ 52.63	$ 56.12	$ 51.68	$ 51.96