Dividends Declared Per Share (Detail) - $ / shares	3 Months Ended		9 Months Ended
Dividends Declared Per Share (Detail) - $ / shares	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Dividends Payable [Line Items]
Dividend Amount	$ 0.45	$ 0.425	$ 1.35	$ 1.275
O2025 M10 Dividends [Member]
Dividends Payable [Line Items]
Declaration Date			Oct. 29, 2025
Date Paid (Payable)			Nov. 28, 2025
Shareholder of Record Date			Nov. 13, 2025
Dividend Amount			$ 0.45
O2025 M7 Dividends [Member]
Dividends Payable [Line Items]
Declaration Date			Jul. 29, 2025
Date Paid (Payable)			Aug. 28, 2025
Shareholder of Record Date			Aug. 14, 2025
Dividend Amount			$ 0.45
O 2025 M4 Dividends [Member]
Dividends Payable [Line Items]
Declaration Date			Apr. 30, 2025
Date Paid (Payable)			May 30, 2025
Shareholder of Record Date			May 15, 2025
Dividend Amount			$ 0.45
O 2025 M1 Dividends [Member]
Dividends Payable [Line Items]
Declaration Date			Jan. 29, 2025
Date Paid (Payable)			Feb. 28, 2025
Shareholder of Record Date			Feb. 13, 2025
Dividend Amount			$ 0.45
O2024 M10 Dividends Member
Dividends Payable [Line Items]
Declaration Date			Oct. 30, 2024
Date Paid (Payable)			Nov. 29, 2024
Shareholder of Record Date			Nov. 14, 2024
Dividend Amount			$ 0.425
O2024 M7 Dividends [Member]
Dividends Payable [Line Items]
Declaration Date			Jul. 31, 2024
Date Paid (Payable)			Aug. 29, 2024
Shareholder of Record Date			Aug. 15, 2024
Dividend Amount			$ 0.425
O2024 M5 Dividends [Member]
Dividends Payable [Line Items]
Declaration Date			May 01, 2024
Date Paid (Payable)			May 31, 2024
Shareholder of Record Date			May 16, 2024
Dividend Amount			$ 0.425
O2024 M1 Dividends Member
Dividends Payable [Line Items]
Declaration Date			Jan. 31, 2024
Date Paid (Payable)			Feb. 29, 2024
Shareholder of Record Date			Feb. 14, 2024
Dividend Amount			$ 0.425

UNITIL CORP 10-Q