Consolidated Statements of Income and Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Operating revenue	$ 389,225	$ 339,706	$ 305,082
Production Expenses:
Purchased water	86,456	72,971	61,089
Power	7,295	6,102	6,121
Groundwater extraction charges	47,817	32,088	31,240
Other production expenses	15,203	13,167	12,178
Total production expenses	156,771	124,328	110,628
Administrative and general	55,011	48,038	47,131
Maintenance	17,430	17,476	14,956
Property taxes and other non-income taxes	13,642	12,123	11,667
Depreciation and amortization	48,292	44,625	40,740
Total operating expense	291,146	246,590	225,122
Operating income	98,079	93,116	79,960
Other (expense) income:
Interest on long-term debt	(22,610)	(20,205)	(20,925)
Mortgage and other interest expense	(319)	(1,633)	(1,261)
Gain on sale of equity interests in Texas Water Alliance Limited and utility property	12,499	0	0
Gain on sale of real estate investments	6,903	10,419	1,886
Gain on sale of California Water Service Group stock	0	3,197	0
Dividend income	75	87	174
Other, net	1,866	1,400	1,320
Income before income taxes	96,493	86,381	61,154
Provision for income taxes	35,393	33,542	23,272
Net income before noncontrolling interest	61,100	52,839	37,882
Less net income attributable to the noncontrolling interest	1,896	0	0
SJW Group net income	59,204	52,839	37,882
Other comprehensive income (loss):
Unrealized income (loss) on investment, net of taxes of $466 in 2017, $657 in 2016, and ($141) in 2015	679	955	(206)
Reclassification adjustment for gain realized on investment, net of taxes of $1,198 in 2016	0	(1,742)	0
SJW Group comprehensive income	$ 59,883	$ 52,052	$ 37,676
SJW Group earnings per share
—Basic (in dollars per share)	$ 2.89	$ 2.59	$ 1.86
—Diluted (in dollars per share)	$ 2.86	$ 2.57	$ 1.85
Weighted average shares outstanding
—Basic (shares)	20,506,960	20,439,957	20,360,663
—Diluted (shares)	20,685,118	20,588,973	20,515,643