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Details on Long Term Debt (Parenthetical) (Detail) |  | 12 Months Ended |
--- | --- | --- | ---
Nov. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Aug. 01, 2016
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% | 3.70%
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date | 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% |
Debt instrument due date | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date | 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% |
Debt instrument due date | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date |  | 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% |
Debt instrument due date | 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% |
Debt instrument due date |  | Aug. 01, 2017 | Aug. 01, 2017 |

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