Summary of Significant Accounting Policies (Regulatory Rate Filings) (Details)													3 Months Ended		9 Months Ended	12 Months Ended
	Feb. 01, 2020 USD ($)	Jan. 31, 2020 USD ($)	Jan. 20, 2020 USD ($)	Dec. 20, 2019 USD ($)	Nov. 20, 2019 USD ($)	Oct. 01, 2019 USD ($)	Sep. 30, 2019 USD ($)	Aug. 30, 2019 USD ($)	Feb. 01, 2019 USD ($)	Jan. 28, 2019 USD ($)	Jan. 01, 2019 USD ($)	Sep. 14, 2018 USD ($)	Dec. 31, 2019 USD ($) mi²	Mar. 31, 2019	Dec. 31, 2019 USD ($) mi²	Dec. 31, 2019 USD ($) mi²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 07, 2020	Jan. 28, 2020	Dec. 06, 2019 USD ($)	Oct. 18, 2019 USD ($)	Aug. 01, 2019 USD ($)	Jul. 26, 2019 USD ($)	Feb. 08, 2019 USD ($)	Jan. 04, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Regulatory Balancing and Memorandum Accounts, Net Under-Collected													$ 1,428,000		$ 1,428,000	$ 1,428,000
Balancing and Memorandum Cost Recovery Account, Refunds (Collections)																(2,603,000)	$ 1,000
Balancing and Memorandum Revenue Account, Increase (Decrease)																(4,984,000)	3,827,000
Revenue from contracts with customers																$ 447,720,000	389,302,000	$ 381,777,000
Minimum [Member]
Public Utilities, General Disclosures [Line Items]
Service area in square miles | mi²													246		246	246
CPED Proposed Penalty, Per Offense, Per Day												$ 500
Maximum [Member]
Public Utilities, General Disclosures [Line Items]
CPED Proposed Penalty, Per Offense, Per Day												50,000
2018 WCMA [Member]
Public Utilities, General Disclosures [Line Items]
Balancing and Memorandum Revenue Account, Increase (Decrease)																$ (9,386,000)	$ 9,386,000
San Jose Water Company [Member]
Public Utilities, General Disclosures [Line Items]
CPED Calculated Double-Billing Refund												4,935,000
San Jose Water Company [Member] | General Rate Case No. 18-01-004 [Member]
Public Utilities, General Disclosures [Line Items]
Requested Rate Increase, Year One																										$ 34,288,000
Requested Rate Increase, Year One, Percentage																										9.76%
Requested Rate Increase, Year Two																										$ 14,232,000
Requested Rate Increase, Year Two, Percentage																										3.70%
Requested Rate Increase, Year Three																										$ 20,582,000
Requested Rate Increase, Year Three, Percentage																										5.17%
Requested Balancing and Memorandum Accounts, net																										$ 20,725,000
San Jose Water Company [Member] | Advice Letter No. 528/528A [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount											$ 16,378,000
Public Utilities, Approved Rate Increase (Decrease), Percentage											4.55%
Balancing and Memorandum Cost Recovery Account, Refunds (Collections)											$ 27,045,000
San Jose Water Company [Member] | Advice Letter No. 530 [Member]
Public Utilities, General Disclosures [Line Items]
Approved Service Charge Refund																									$ 2,020,000
San Jose Water Company [Member] | CPED Calculated Refund 2014-2016 [Member]
Public Utilities, General Disclosures [Line Items]
CPED Calculated Service Charge Refund												2,061,000
San Jose Water Company [Member] | CPED Calculated Refund 1987-2013 [Member]
Public Utilities, General Disclosures [Line Items]
CPED Calculated Service Charge Refund												$ 1,990,000
San Jose Water Company [Member] | OII No. 18-09-003 Settlement Agreement [Member]
Public Utilities, General Disclosures [Line Items]
Utility Plant Investment Commitment																								$ 5,000,000
San Jose Water Company [Member] | OII No. 18-09-003 Settlement Agreement [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Customer Credit Refund																								$ 2,100,000
San Jose Water Company [Member] | Advice Letter No. 531 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Reimbursement Fee, Percentage														1.40%	1.23%
San Jose Water Company [Member] | Advice Letter No. 532 [Member] | 2018 WCMA [Member]
Public Utilities, General Disclosures [Line Items]
Balancing and Memorandum Revenue Account, Increase (Decrease)																9,386,000
San Jose Water Company [Member] | Advice Letter No. 532 [Member] | 2019 WCMA [Member]
Public Utilities, General Disclosures [Line Items]
Balancing and Memorandum Revenue Account, Increase (Decrease)																$ (639,000)
San Jose Water Company [Member] | Advice Letter No. 537 [Member]
Public Utilities, General Disclosures [Line Items]
Memorandum Account, Requested Recovery (Refund), Monthly Amount																						$ (20.84)
San Jose Water Company [Member] | Advice Letter No. 541 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 8,600,000
Public Utilities, Requested Rate Increase (Decrease), Percentage					2.28%
San Jose Water Company [Member] | Application No. 19-12-002 [Member]
Public Utilities, General Disclosures [Line Items]
Requested Rate Increase, Year One																					$ 2,300,000
Requested Rate Increase, Year One, Percentage																					0.61%
Requested Rate Increase, Year Two																					$ 4,000,000.0
Requested Rate Increase, Year Two, Percentage																					1.04%
Requested Rate Increase, Year Three																					$ 2,500,000
Requested Rate Increase, Year Three, Percentage																					0.65%
Regulatory Rate Filings, Requested Rate Increase, Year Four																					$ 300,000
Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Four																					0.09%
San Jose Water Company [Member] | Advice Letter No. 534 [Member]
Public Utilities, General Disclosures [Line Items]
Memorandum Account, Requested Recovery (Refund)																							$ 1,243,000
San Jose Water Company [Member] | Advice Letter No. 535 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount								$ 700,000
Public Utilities, Requested Rate Increase (Decrease), Percentage								0.17%
Connecticut Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Authorized WICA Surcharge, Percentage													3.24%		3.24%	3.24%
Connecticut Water Company [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Requested WICA Surcharge, Percentage																			5.75%	5.84%
Requested WICA Surcharge Increase (Decrease), Percentage																			(0.09%)	2.60%
Connecticut Water Company & Heritage Valley Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Authorized WRA Revenue													$ 18,588,000
Revenue from contracts with customers													20,428,000
WRA Adjustment to Revenue													$ (1,840,000)
Avon Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Authorized WICA Surcharge, Percentage													9.31%		9.31%	9.31%
SJWTX, Inc. [Member]
Public Utilities, General Disclosures [Line Items]
Authorized Regulatory Surcharge, Per Thousand Gallons									$ 1.05	$ 1.13
Deer Creek Ranch [Member]
Public Utilities, General Disclosures [Line Items]
Authorized Regulatory Surcharge, Per Thousand Gallons						$ 2.02	$ 2.19
Authorized Regulatory Base Charge, Per Thousand Gallons						$ 3.04
Canyon Lake [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Authorized Regulatory Surcharge, Per Thousand Gallons	$ 0.95	$ 1.05
Maine Water Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 221,000
Public Utilities, Requested Rate Increase (Decrease), Percentage				14.70%
Maine Water Company [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 371
Maine Water Company [Member] | Minimum [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Requested WISC Surcharge Increase (Decrease), Percentage			1.76%
Maine Water Company [Member] | Maximum [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Requested WISC Surcharge Increase (Decrease), Percentage			3.00%