v3.25.2
DEBT AND FINANCING ARRANGEMENTS - Details On Long Term Debt (Parenthetical) (Detail)
6 Months Ended
Jun. 30, 2025
3.70% Senior Notes, Due August 1, 2026  
Debt Instrument [Line Items]  
Stated interest rate 3.70%
Debt instrument due date Aug. 01, 2026
3.43% Senior Notes, Due December 18, 2029  
Debt Instrument [Line Items]  
Stated interest rate 3.43%
Debt instrument due date Dec. 18, 2029
5.99% Senior Notes, Due August 21, 2034  
Debt Instrument [Line Items]  
Stated interest rate 5.99%
Debt instrument due date Aug. 21, 2034
First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028 | Unitil Energy Systems Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 6.96%
Debt instrument due date Sep. 01, 2028
First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031 | Unitil Energy Systems Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 8.00%
Debt instrument due date May 01, 2031
First Mortgage Bonds 6.32% Senior Secured Notes, Due September 15, 2036 | Unitil Energy Systems Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 6.32%
Debt instrument due date Sep. 15, 2036
3.58% Senior Secured Notes, Due September 15, 2040 | Unitil Energy Systems Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 3.58%
Debt instrument due date Sep. 15, 2040
First Mortgage Bonds 4.18% Senior Secured Notes Due November 30, 2048 | Unitil Energy Systems Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 4.18%
Debt instrument due date Nov. 30, 2048
5.69% Senior Secured Notes, Due August 21, 2054 | Unitil Energy Systems Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.69%
Debt instrument due date Aug. 21, 2054
3.52% Senior Notes, Due November 1, 2027 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 3.52%
Debt instrument due date Nov. 01, 2027
3.52% Senior Notes, Due November 1, 2027 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 3.52%
Debt instrument due date Nov. 01, 2027
7.37% Notes, Due January 15, 2029 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 7.37%
Debt instrument due date Jan. 15, 2029
5.90% Notes, Due December 15, 2030 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.90%
Debt instrument due date Dec. 15, 2030
7.98% Notes, Due June 1, 2031 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 7.98%
Debt instrument due date Jun. 01, 2031
5.70% Notes, Due July 2, 2033 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.70%
Debt instrument due date Jul. 02, 2033
5.54% Senior Notes, Due August 21, 2034 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.54%
Debt instrument due date Aug. 21, 2034
5.54% Senior Notes, Due August 21, 2034 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.54%
Debt instrument due date Aug. 21, 2034
3.78% Senior Notes, Due September 15, 2040 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 3.78%
Debt instrument due date Sep. 15, 2040
3.78% Senior Notes, Due September 15, 2040 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 3.78%
Debt instrument due date Sep. 15, 2040
5.99% Senior Notes, Due August 21, 2044 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.99%
Debt instrument due date Aug. 21, 2044
4.32% Senior Notes, Due November 1, 2047 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 4.32%
Debt instrument due date Nov. 01, 2047
4.32% Senior Notes, Due November 1, 2047 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 4.32%
Debt instrument due date Nov. 01, 2047
5.96% Senior Notes, Due July 2, 2053 | Fitchburg Gas And Electric Light Company [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.96%
Debt instrument due date Jul. 02, 2053
7.72% Senior Notes, Due December 3, 2038 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 7.72%
Debt instrument due date Dec. 03, 2038
5.74% Senior Notes, Due August 21, 2039 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.74%
Debt instrument due date Aug. 21, 2039
5.74% Senior Notes, Due August 21, 2034 | Granite State Gas Transmission Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 5.74%
Debt instrument due date Aug. 21, 2034
4.42% Senior Notes, Due October 15, 2044 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 4.42%
Debt instrument due date Oct. 15, 2044
4.04% Senior Notes, Due September 12, 2049 | Northern Utilities Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 4.04%
Debt instrument due date Sep. 12, 2049
3.72% Senior Notes, Due November 1, 2027 | Granite State Gas Transmission Inc [Member]  
Debt Instrument [Line Items]  
Stated interest rate 3.72%
Debt instrument due date Nov. 01, 2027
2.64% Senior Secured Notes, Due December 18, 2030 | Unitil Realty Corp [Member]  
Debt Instrument [Line Items]  
Stated interest rate 2.64%
Debt instrument due date Dec. 18, 2030