Commitments Narrative (Details) $ in Thousands			12 Months Ended
	Nov. 07, 2015 gal	May 19, 2003 gal	Dec. 31, 2019 USD ($) employee acre_foot $ / yr $ / million_gallons $ / Ccf gal	Dec. 31, 2018 USD ($) gal	Dec. 31, 2017 USD ($) gal	Dec. 31, 2008 $ / yr	Dec. 31, 1997	Jan. 08, 2020 USD ($)	Nov. 30, 2016	Dec. 31, 2008 $ / million_gallons
Long-term Purchase Commitment [Line Items]
Document Period End Date			Dec. 31, 2019
Purchased water			$ 99,118	$ 97,378	$ 86,456
Reduction in delivery schedule			10.00%
Intangible assets			$ 33,424	$ 15,799
Collective bargaining agreement percent of wage increase for 2017									3.00%
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			21,862,000,000	21,345,000,000	20,857,000,000
Purchased water			$ 96,285	$ 87,702	$ 78,703
Minimum percent of delivery schedule to be purchased in 2017			90.00%
Minimum volume of water to be purchased in 2017 | gal			19,794,000,000
Cost of water to be purchased in 2017			$ 89,539
Current contract water rate | $ / million_gallons			4.5
Service Agreements [Member]
Long-term Purchase Commitment [Line Items]
Contract life of agreement with City of Cupertino							25 years
Intangible assets			$ 4,128
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			361
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
SJWTX,Inc. dba Canyon Lake Water Supply Corporation [Member] | Lower Colorado River Authority [Member]
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment, Minimum Area Required, Next Twelve Months | acre_foot			250
RWA
Long-term Purchase Commitment [Line Items]
Purchase commitment period						50 years
RWA | Capacity Agreement
Long-term Purchase Commitment [Line Items]
Current contract water rate						75				2,621
Purchase commitment period						14 years
MDC | Capacity Agreement
Long-term Purchase Commitment [Line Items]
Purchase commitment period		50 years
Agreement to purchase water | gal		283,000,000
Kennebec Water District
Long-term Purchase Commitment [Line Items]
Current contract water rate | $ / yr			5,000
Tariff rate | $ / Ccf			1.51
Purchase commitment period	5 years
Agreement to purchase water | gal	60,000,000
Executive, Administrative or Supervisory Personnel [Member] | SJW Group [Member]
Long-term Purchase Commitment [Line Items]
Number of employees | employee			136
Union Member [Member] | SJW Group [Member]
Long-term Purchase Commitment [Line Items]
Number of employees | employee			225
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
Subsequent Event [Member] | Concession Fees [Member]
Long-term Purchase Commitment [Line Items]
Intangible assets								$ 5,000