Capitalization - Narrative (Details) $ / shares in Units, gal in Millions		1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	May 12, 2025 USD ($)	Sep. 30, 2025 USD ($) shares	May 31, 2024 USD ($) loan	Sep. 30, 2025 USD ($) $ / shares shares	Jun. 30, 2025 USD ($) shares	Sep. 30, 2025 USD ($) project $ / shares shares gal	Sep. 30, 2024 USD ($) shares	Sep. 30, 2025 USD ($)	Oct. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)	Apr. 30, 2023 shares
Class of Stock [Line Items]
Value of common shares issued				$ 16,992,000	$ 3,604,000
Number of shares approved to issue and sell (in shares) \| shares											1,000,000.0
Total net proceeds from shares issued and sold under agreement						$ 21,281,000	$ 740,000
Debt drawn down						3,603,000	592,000
Loans Payable \| New Jersey SRF
Class of Stock [Line Items]
Borrowing amount		$ 300,000,000.0		300,000,000.0		300,000,000.0		$ 300,000,000.0
Maximum petitioned borrowing amount approved		260,000,000.0		260,000,000.0		260,000,000.0		260,000,000.0
Loans Payable \| September 2024 Delaware SRF \| Tidewater
Class of Stock [Line Items]
Borrowing amount							$ 2,200,000
Interest rate							0.00%
Debt drawn down								1,700,000
Loans Payable \| May 2024 Delaware SRF \| Tidewater
Class of Stock [Line Items]
Borrowing amount			$ 5,600,000
Interest rate			2.00%
Debt drawn down						900,000
Number of debt instruments \| loan			4
Loans Payable \| 2023 Delaware SRF \| Tidewater
Class of Stock [Line Items]
Debt drawn down						$ 7,200,000
Number of active construction projects funded by debt instruments \| project						2
Carrying amount of debt		8,300,000		8,300,000		$ 8,300,000		8,300,000
Size of elevated storage tank (in gallons) \| gal						1
Loans Payable \| Amortizing Secured Notes And State Revolving Trust Notes
Class of Stock [Line Items]
Carrying amount of debt		230,500,000		230,500,000		$ 230,500,000		230,500,000		$ 229,800,000
Loans Payable \| Construction Advances
Class of Stock [Line Items]
Carrying amount of debt		$ 24,100,000		$ 24,100,000		$ 24,100,000		24,100,000		$ 22,600,000
Secured Debt \| 5.99% FMBs due 2055 \| Subsequent Event
Class of Stock [Line Items]
Borrowing amount									$ 30,000,000
Interest rate									5.99%
Underwritten Public Offering and/or ATM Offering
Class of Stock [Line Items]
Maximum number of shares petitioned to issue and sell during offering (in shares) \| shares		2,500,000
ATM Equity Sales Agreement
Class of Stock [Line Items]
Aggregate gross sales price	$ 110,000,000.0
Number of shares issued and sold under agreement (in shares) \| shares				316,786		380,741
Weighted average price per share of shares issued and sold under agreement (in dollars per share) \| $ / shares				$ 53.47		$ 54.10
Total net proceeds from shares issued and sold under agreement				$ 16,700,000		$ 20,300,000
Aggregate gross sales price of shares remaining to be issued under agreement		$ 89,400,000		$ 89,400,000		$ 89,400,000		$ 89,400,000
Common Stock
Class of Stock [Line Items]
Number of common shares issued (in shares) \| shares				317,000	64,000	12,522	13,498
Value of common shares issued				$ 16,992,000	$ 3,604,000	$ 700,000	$ 700,000

MIDDLESEX WATER CO 10-Q