Details on Long Term Debt (Parenthetical) (Detail)		3 Months Ended		12 Months Ended
	Nov. 01, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
6.33% Senior Notes, Due May 1, 2022
Debt Instrument [Line Items]
Stated interest rate		6.33%	6.33%	6.33%
Debt instrument due date		May 01, 2022	May 01, 2022	May 01, 2022
3.70% Senior Notes, Due August 1, 2026
Debt Instrument [Line Items]
Stated interest rate		3.70%	3.70%	3.70%
Debt instrument due date		Aug. 01, 2026	Aug. 01, 2026	Aug. 01, 2026
First Mortgage Bonds 5.24% Senior Secured Notes, Due March 2, 2020 | Unitil Energy Systems Inc
Debt Instrument [Line Items]
Stated interest rate		5.24%	5.24%	5.24%
Debt instrument due date		Mar. 02, 2020	Mar. 02, 2020	Mar. 02, 2020
First Mortgage Bonds 8.49% Senior Secured Notes, Due October 14, 2024 | Unitil Energy Systems Inc
Debt Instrument [Line Items]
Stated interest rate		8.49%	8.49%	8.49%
Debt instrument due date		Oct. 14, 2024	Oct. 14, 2024	Oct. 14, 2024
First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028 | Unitil Energy Systems Inc
Debt Instrument [Line Items]
Stated interest rate		6.96%	6.96%	6.96%
Debt instrument due date		Sep. 01, 2028	Sep. 01, 2028	Sep. 01, 2028
First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031 | Unitil Energy Systems Inc
Debt Instrument [Line Items]
Stated interest rate		8.00%	8.00%	8.00%
Debt instrument due date		May 01, 2031	May 01, 2031	May 01, 2031
First Mortgage Bonds 6.32% Senior Secured Notes, Due September 15, 2036 | Unitil Energy Systems Inc
Debt Instrument [Line Items]
Stated interest rate		6.32%	6.32%	6.32%
Debt instrument due date		Sep. 15, 2036	Sep. 15, 2036	Sep. 15, 2036
6.75% Senior Notes, Due November 30, 2023 | Fitchburg Gas and Electric Light Company
Debt Instrument [Line Items]
Stated interest rate		6.75%	6.75%	6.75%
Debt instrument due date		Nov. 30, 2023	Nov. 30, 2023	Nov. 30, 2023
6.79% Senior Notes, Due October 15, 2025 | Fitchburg Gas and Electric Light Company
Debt Instrument [Line Items]
Stated interest rate		6.79%	6.79%	6.79%
Debt instrument due date		Oct. 15, 2025	Oct. 15, 2025	Oct. 15, 2025
3.52% Senior Notes, Due November 1, 2027 | Fitchburg Gas and Electric Light Company
Debt Instrument [Line Items]
Stated interest rate	3.52%	3.52%	3.52%	3.52%
Debt instrument due date	Nov. 01, 2027	Nov. 01, 2027	Nov. 01, 2027	Nov. 01, 2027
3.52% Senior Notes, Due November 1, 2027 | Northern Utilities Inc
Debt Instrument [Line Items]
Stated interest rate	3.52%	3.52%	3.52%	3.52%
Debt instrument due date	Nov. 01, 2027	Nov. 01, 2027	Nov. 01, 2027	Nov. 01, 2027
7.37% Senior Notes, Due January 15, 2029 | Fitchburg Gas and Electric Light Company
Debt Instrument [Line Items]
Stated interest rate		7.37%	7.37%	7.37%
Debt instrument due date		Jan. 15, 2029	Jan. 15, 2029	Jan. 15, 2029
5.90% Senior Notes, Due December 15, 2030 | Fitchburg Gas and Electric Light Company
Debt Instrument [Line Items]
Stated interest rate		5.90%	5.90%	5.90%
Debt instrument due date		Dec. 15, 2030	Dec. 15, 2030	Dec. 15, 2030
7.98% Senior Notes, Due June 1, 2031 | Fitchburg Gas and Electric Light Company
Debt Instrument [Line Items]
Stated interest rate		7.98%	7.98%	7.98%
Debt instrument due date		Jun. 01, 2031	Jun. 01, 2031	Jun. 01, 2031
4.32% Senior Notes, Due November 1, 2047 | Fitchburg Gas and Electric Light Company
Debt Instrument [Line Items]
Stated interest rate	4.32%	4.32%	4.32%	4.32%
Debt instrument due date	Nov. 01, 2047	Nov. 01, 2047	Nov. 01, 2047	Nov. 01, 2047
4.32% Senior Notes, Due November 1, 2047 | Northern Utilities Inc
Debt Instrument [Line Items]
Stated interest rate	4.32%	4.32%	4.32%	4.32%
Debt instrument due date	Nov. 01, 2047	Nov. 01, 2047	Nov. 01, 2047	Nov. 01, 2047
6.95% Senior Notes, Due December 3, 2018 | Northern Utilities Inc
Debt Instrument [Line Items]
Stated interest rate		6.95%	6.95%	6.95%
Debt instrument due date		Dec. 03, 2018	Dec. 03, 2018	Dec. 03, 2018
5.29% Senior Notes, Due March 2, 2020 | Northern Utilities Inc
Debt Instrument [Line Items]
Stated interest rate		5.29%	5.29%	5.29%
Debt instrument due date		Mar. 02, 2020	Mar. 02, 2020	Mar. 02, 2020
7.72% Senior Notes, Due December 3, 2038 | Northern Utilities Inc
Debt Instrument [Line Items]
Stated interest rate		7.72%	7.72%	7.72%
Debt instrument due date		Dec. 03, 2038	Dec. 03, 2038	Dec. 03, 2038
4.42% Senior Notes, Due October 15, 2044 | Northern Utilities Inc
Debt Instrument [Line Items]
Stated interest rate		4.42%	4.42%	4.42%
Debt instrument due date		Oct. 15, 2044	Oct. 15, 2044	Oct. 15, 2044
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc
Debt Instrument [Line Items]
Stated interest rate		7.15%	7.15%	7.15%
Debt instrument due date		Dec. 15, 2018	Dec. 15, 2018	Dec. 15, 2018
3.72% Senior Notes, Due November 1, 2027 | Granite State Gas Transmission Inc
Debt Instrument [Line Items]
Stated interest rate	3.72%	3.72%	3.72%	3.72%
Debt instrument due date	Nov. 01, 2027	Nov. 01, 2027	Nov. 01, 2027	Nov. 01, 2027
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp
Debt Instrument [Line Items]
Stated interest rate		8.00%	8.00%	8.00%
Debt instrument due date		Aug. 01, 2017	Aug. 01, 2017	Aug. 01, 2017