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Details on Long Term Debt (Parenthetical) (Detail) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015
6.33% 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
First Mortgage Bonds 5.24% Series, 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% Series, 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% Series, 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% Series, 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% Series, 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% 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
7.37% 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
7.98% 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
6.79% 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
5.90% 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
6.95% Senior Notes, Series A, 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, Series B, 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
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

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