Unaudited Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenue	$ 93,529	$ 124,578	$ 102,073	$ 69,045	$ 79,306	$ 112,344	$ 86,944	$ 61,112	$ 389,225	$ 339,706	$ 305,082
Operating Income (Loss)	18,214	38,245	30,787	10,833	16,389	36,346	29,800	10,581	98,079	93,116	79,960
Net income	17,305	19,540	18,688	3,671	13,727	18,959	16,775	3,378	59,204	52,839	37,882
Comprehensive income	$ 17,732	$ 19,620	$ 18,744	$ 3,787	$ 13,834	$ 18,790	$ 15,520	$ 3,908	$ 59,883	$ 52,052	$ 37,676
SJW Group earnings per share
—Basic (in dollars per share)	$ 0.84	$ 0.95	$ 0.91	$ 0.18	$ 0.67	$ 0.93	$ 0.82	$ 0.17	$ 2.89	$ 2.59	$ 1.86
—Diluted (in dollars per share)	0.84	0.94	0.90	0.18	0.67	0.92	0.82	0.16	$ 2.86	$ 2.57	$ 1.85
Dividends per share	0.38	0.22	0.22	0.22	0.20	0.20	0.20	0.20
--- High (in dollars per share) [Member]
SJW Group earnings per share
Market Price, Stock	68.13	57.43	53.00	55.30	56.69	46.66	39.38	37.50
--- Low (in dollars per share) [Member]
SJW Group earnings per share
Market Price, Stock	$ 57.60	$ 48.46	$ 45.74	$ 46.13	$ 42.19	$ 38.13	$ 32.66	$ 29.35