Debt and Financing Arrangements - Details On Long Term Debt (Parenthetical) (Detail)	6 Months Ended
Jun. 30, 2026
Debt Instrument [Line Items]
Debt instrument due date	Oct. 31, 2026
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
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
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
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
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
5.62% Senior Notes, Due April 30, 2036 \| Fitchburg Gas And Electric Light Company [Member]
Debt Instrument [Line Items]
Stated interest rate	5.62%
Debt instrument due date	Apr. 30, 2036
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
5.87% Senior Notes, Due April 30, 2041 \| Fitchburg Gas And Electric Light Company [Member]
Debt Instrument [Line Items]
Stated interest rate	5.87%
Debt instrument due date	Apr. 30, 2041
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
5.70% Senior Notes, Due July 8, 2030 \| Bangor Natural Gas [Member]
Debt Instrument [Line Items]
Stated interest rate	5.70%
Debt instrument due date	Jul. 08, 2030
6.31% Senior Notes, Due July 8, 2035 \| Bangor Natural Gas [Member]
Debt Instrument [Line Items]
Stated interest rate	6.31%
Debt instrument due date	Jul. 08, 2035
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
5.72% Senior Secured Notes, Due June 1, 2028 \| AWC-NH [Member]
Debt Instrument [Line Items]
Stated interest rate	5.72%
Debt instrument due date	Jun. 01, 2028
3.17% Senior Notes, Due June 1, 2051 \| Abenaki [Member]
Debt Instrument [Line Items]
Stated interest rate	3.17%
Debt instrument due date	Jun. 01, 2051
1.296% - 2.864% Senior Notes, Due October 1, 2031 - May 1, 2045 \| Abenaki [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Stated interest rate	2.864%
Debt instrument due date	May 01, 2045
1.296% - 2.864% Senior Notes, Due October 1, 2031 - May 1, 2045 \| Abenaki [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Stated interest rate	1.296%
Debt instrument due date	Oct. 01, 2031