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v3.20.4

Details on Long Term Debt (Parenthetical) (Detail) |  | 12 Months Ended
--- | --- | ---
Sep. 12, 2019 | Dec. 31, 2020 | Dec. 31, 2019
Northern Utilities Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 4.04% |  |
6.33% Senior Notes, Due May 1, 2022 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 6.33% | 6.33%
Debt instrument due date |  | 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%
Debt instrument due date |  | Aug. 01, 2026 | Aug. 01, 2026
3.43% Senior Notes, Due December 18, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 3.43% | 3.43%
Debt instrument due date |  | Dec. 18, 2029 | Dec. 18, 2029
First Mortgage Bonds 5.24% Senior Secured Notes, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Mar. 02, 2020
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%
Debt instrument due date |  | Mar. 02, 2020 |
First Mortgage Bonds 8.49% Senior Secured Notes, Due October 14, 2024 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Oct. 14, 2024
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%
Debt instrument due date |  | Oct. 14, 2024 |
First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Sep. 01, 2028
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%
Debt instrument due date |  | Sep. 01, 2028 |
First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | May 01, 2031
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%
Debt instrument due date |  | May 01, 2031 |
First Mortgage Bonds 6.32% Senior Secured Notes, Due September 15, 2036 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Sep. 15, 2036
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%
Debt instrument due date |  | Sep. 15, 2036 |
First Mortgage Bonds 3.58% Senior Secured Notes Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Sep. 15, 2040
First Mortgage Bonds 3.58% Senior Secured Notes Due September 15, 2040 | Unitil Energy Systems Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 3.58% | 3.58%
Debt instrument due date |  | Sep. 15, 2040 |
First Mortgage Bonds 4.18% Senior Secured Notes Due November 30, 2048 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Nov. 30, 2048
First Mortgage Bonds 4.18% Senior Secured Notes Due November 30, 2048 | Unitil Energy Systems Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 4.18% | 4.18%
Debt instrument due date |  | Nov. 30, 2048 |
6.75% Senior Notes, Due November 30, 2023 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Nov. 30, 2023
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%
Debt instrument due date |  | Nov. 30, 2023 |
6.79% Senior Notes, Due October 15, 2025 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Oct. 15, 2025
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%
Debt instrument due date |  | Oct. 15, 2025 |
3.52% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Nov. 01, 2027
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%
Debt instrument due date |  | 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%
Debt instrument due date |  | Nov. 01, 2027 |
7.37% Notes, Due January 15, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Jan. 15, 2029
7.37% Notes, Due January 15, 2029 | Fitchburg Gas and Electric Light Company |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 7.37% | 7.37%
Debt instrument due date |  | Jan. 15, 2029 |
5.90% Notes, Due December 15, 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Dec. 15, 2030
5.90% Notes, Due December 15, 2030 | Fitchburg Gas and Electric Light Company |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 5.90% | 5.90%
Debt instrument due date |  | Dec. 15, 2030 |
7.98% Notes, Due June 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Jun. 01, 2031
7.98% Notes, Due June 1, 2031 | Fitchburg Gas and Electric Light Company |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 7.98% | 7.98%
Debt instrument due date |  | Jun. 01, 2031 |
3.78% Senior Notes, Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Sep. 15, 2040
3.78% Senior Notes, Due September 15, 2040 | Fitchburg Gas and Electric Light Company |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 3.78% | 3.78%
Debt instrument due date |  | Sep. 15, 2040 |
3.78% Senior Notes, Due September 15, 2040 | Northern Utilities Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 3.78% | 3.78%
Debt instrument due date |  | Sep. 15, 2040 |
4.32% Senior Notes, Due November 1, 2047 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Nov. 01, 2047
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%
Debt instrument due date |  | 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%
Debt instrument due date |  | Nov. 01, 2047 |
5.29% Senior Notes, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Mar. 02, 2020
5.29% Senior Notes, Due March 2, 2020 | Northern Utilities Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 5.29% | 5.29%
Debt instrument due date |  | Mar. 02, 2020 |
7.72% Senior Notes, Due December 3, 2038 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Dec. 03, 2038
7.72% Senior Notes, Due December 3, 2038 | Northern Utilities Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 7.72% | 7.72%
Debt instrument due date |  | Dec. 03, 2038 |
4.42% Senior Notes, Due October 15, 2044 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Oct. 15, 2044
4.42% Senior Notes, Due October 15, 2044 | Northern Utilities Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 4.42% | 4.42%
Debt instrument due date |  | Oct. 15, 2044 |
4.04% Senior Notes, Due September 12, 2049 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Sep. 12, 2049
4.04% Senior Notes, Due September 12, 2049 | Northern Utilities Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 4.04% | 4.04%
Debt instrument due date | Sep. 12, 2049 | Sep. 12, 2049 |
3.72% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Debt instrument due date |  |  | Nov. 01, 2027
3.72% Senior Notes, Due November 1, 2027 | Granite State Gas Transmission Inc |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 3.72% | 3.72%
Debt instrument due date |  | Nov. 01, 2027 |
2.64% Senior Notes, Due December 18, 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate |  | 2.64% | 2.64%
Debt instrument due date |  | Dec. 18, 2030 | Dec. 18, 2030

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