Commitments Narrative (Details) $ in Thousands, gal in Millions	1 Months Ended	12 Months Ended
	Nov. 30, 2016	Dec. 31, 2017 USD ($) employee acre_foot $ / million_gallons gal	Dec. 31, 2016 USD ($) gal	Dec. 31, 2015 USD ($) gal	Dec. 31, 1997
Long-term Purchase Commitment [Line Items]
Purchased water		$ 86,456	$ 72,971	$ 61,089
Reduction in delivery schedule		10.00%
Intangible assets		$ 14,413	$ 23,989
Collective Bargaining Agreement, Term of Contract	3 years
Collective bargaining agreement percent of wage increase for 2017	3.50%
Collective bargaining agreement percent of wage increase for 2018	3.00%
Collective bargaining agreement percent of wage increase for 2019	4.00%
Santa Clara Valley Water District [Member]
Long-term Purchase Commitment [Line Items]
Master contract, negotiated term for delivery schedules		3 years
Water purchased in millions of gallons | gal		20,172	18,241	18,482
Purchased water		$ 76,106	$ 61,645	$ 52,553
Minimum percent of delivery schedule to be purchased in 2017		90.00%
Minimum volume of water to be purchased in 2017 | gal		19,748
Cost of water to be purchased in 2017		$ 77,271
Current contract water rate | $ / million_gallons		3.9
Service Agreements [Member]
Long-term Purchase Commitment [Line Items]
Contract life of agreement with City of Cupertino					25 years
Intangible assets		$ 3,771
Concession Fees [Member]
Long-term Purchase Commitment [Line Items]
Intangible assets		$ 6,800	$ 6,800
SJW Group [Member]
Long-term Purchase Commitment [Line Items]
Number of employees | employee		368
SJWTX,Inc. dba Canyon Lake Water Supply Corporation [Member] | Guadalupe-Blanco River Authority [Member]
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment, Minimum Area Required, Next Twelve Months | acre_foot		6,900
Notice period required for contract adjustment		60 days
Executive, Administrative or Supervisory Personnel [Member] | SJW Group [Member]
Long-term Purchase Commitment [Line Items]
Number of employees | employee		134
Union Member [Member] | SJW Group [Member]
Long-term Purchase Commitment [Line Items]
Number of employees | employee		234
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Contract life of agreement with City of Cupertino		5 years
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Contract life of agreement with City of Cupertino		70 years