Summary of Significant Accounting Policies: (Details)		3 Months Ended			9 Months Ended
	Jun. 28, 2023 USD ($)	Sep. 30, 2023 USD ($) state	Mar. 31, 2023 USD ($) shares	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) customer state registrant shares	Sep. 30, 2022 USD ($) shares	Nov. 06, 2023 USD ($)	Jun. 16, 2023 USD ($)	Jan. 13, 2023 USD ($)	Dec. 31, 2022 USD ($)
Summary of significant accounting policies
Number of customers | customer					1,000,000
Number of states in which entity operates | state		10			10
Number of registrants filing combined report | registrant					2
Issuances of Common Shares under stock-based compensation plans (in shares) | shares					18,351	24,612
Debt instrument term	5 years
Incremental expansion of borrowing capacity		$ 75,000,000			$ 75,000,000
Notes payable to banks		236,500,000			236,500,000					$ 22,000,000
Notes payable to banks		37,000,000			37,000,000					255,500,000
Syndicated Revolving Credit Facility member [Member]
Summary of significant accounting policies
Line of credit arrangements		566,000			566,000
Repayments of Lines of Credit	$ 280,000,000
Letter of Credit | Subsequent Event
Summary of significant accounting policies
Maximum borrowing capacity on line of credit							$ 10,000,000
Syndicated Revolving Credit Facility member [Member]
Summary of significant accounting policies
Notes payable to banks		133,500,000			133,500,000
ASUS
Summary of significant accounting policies
Payments to affiliate for corporate office administrative and general costs		1,100,000		$ 1,300,000	$ 3,800,000	$ 4,000,000
ASUS | Contracted services:
Summary of significant accounting policies
Period of fixed price contracts to operate and maintain water systems at various military bases					50 years
BVES
Summary of significant accounting policies
Payments to affiliate for corporate office administrative and general costs		707,000		$ 663,000	$ 2,800,000	$ 2,100,000
Current borrowing capacity								$ 35,000,000
Incremental expansion of borrowing capacity								25,000,000
Maximum borrowing capacity on line of credit								$ 50,000,000
BVES | Syndicated Revolving Credit Facility member [Member]
Summary of significant accounting policies
Notes payable to banks		37,000,000			$ 37,000,000
BVES | Electric:
Summary of significant accounting policies
Number of customers | customer					24,700
GSWC
Summary of significant accounting policies
Issuances of Common Shares under stock-based compensation plans (in shares) | shares			1		1
Issuance of common share to parent			$ 10,000,000		$ 10,000,000
Intercompany borrowings payment term					24 months
Notes payable to banks		0			$ 0					$ 129,000,000
GSWC | Syndicated Revolving Credit Facility member [Member]
Summary of significant accounting policies
Line of credit arrangements		802,000			802,000
GSWC | Letter of Credit | Subsequent Event
Summary of significant accounting policies
Maximum borrowing capacity on line of credit							20,000,000
GSWC | Syndicated Revolving Credit Facility member [Member]
Summary of significant accounting policies
Notes payable to banks		103,000,000			$ 103,000,000
GSWC | Unsecured Private Placement Notes
Summary of significant accounting policies
Face amount of debt									$ 130,000,000
GSWC | Water:
Summary of significant accounting policies
Number of customers | customer					264,000
Patuxent River Utility Services LLC ( PRUS) | Contracted services:
Summary of significant accounting policies
Period of fixed price contracts to operate and maintain water systems at various military bases					50 years
Initial price contract value		$ 349,000,000
Period of transition to operate and maintain water system at new military base		6 months
Bay State Utility Service LLC (" BSUS") | Contracted services:
Summary of significant accounting policies
Period of fixed price contracts to operate and maintain water systems at various military bases					15 years
Initial price contract value		$ 45,000,000
Contract initial order value to perform an evaluation, construction, and transition services.					$ 2,300,000
Maximum | Syndicated Revolving Credit Facility member [Member]
Summary of significant accounting policies
Current borrowing capacity		$ 150,000,000			$ 150,000,000
Ratio of Indebtedness to Net Capital		0.65			0.65
Maximum | Syndicated Revolving Credit Facility member [Member] | Subsequent Event
Summary of significant accounting policies
Current borrowing capacity							$ 165,000,000
Maximum | ASUS
Summary of significant accounting policies
Period of fixed price contracts to operate and maintain water systems at various military bases					50 years
Maximum | GSWC | Syndicated Revolving Credit Facility member [Member]
Summary of significant accounting policies
Current borrowing capacity		$ 200,000,000			$ 200,000,000