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Details on Long Term Debt (Parenthetical) (Detail) | Mar. 31, 2014 | Dec. 31, 2013 | Mar. 31, 2013
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6.33% Notes, Due May 1, 2022 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.33% | 6.33% | 6.33%
Unitil Energy Systems Inc | First Mortgage Bonds 5.24% Series, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 5.24% | 5.24% | 5.24%
Unitil Energy Systems Inc | First Mortgage Bonds 8.49% Series, Due October 14, 2024 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 8.49% | 8.49% | 8.49%
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Series, Due September 1, 2028 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.96% | 6.96% | 6.96%
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Series, Due May 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 8.00% | 8.00% | 8.00%
Unitil Energy Systems Inc | First Mortgage Bonds 6.32% Series, Due September 15, 2036 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.32% | 6.32% | 6.32%
Fitchburg Gas and Electric Light Company | 6.75% Notes, Due November 30, 2023 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.75% | 6.75% | 6.75%
Fitchburg Gas and Electric Light Company | 7.37% Notes, Due January 15, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.37% | 7.37% | 7.37%
Fitchburg Gas and Electric Light Company | 7.98% Notes, Due June 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.98% | 7.98% | 7.98%
Fitchburg Gas and Electric Light Company | 6.79% Notes, Due October 15, 2025 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.79% | 6.79% | 6.79%
Fitchburg Gas and Electric Light Company | 5.90% Notes, Due December 15, 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 5.90% | 5.90% | 5.90%
Northern Utilities Inc | 6.95% Senior Notes, Series A, Due December 3, 2018 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.95% | 6.95% | 6.95%
Northern Utilities Inc | 5.29% Senior Notes, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 5.29% | 5.29% | 5.29%
Northern Utilities Inc | 7.72% Senior Notes, Series B, Due December 3, 2038 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.72% | 7.72% | 7.72%
Granite State Gas Transmission Inc | 7.15% Senior Notes, Due December 15, 2018 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.15% | 7.15% | 7.15%
Unitil Realty Corp | 8.00% Senior Secured Notes, Due Through August 1, 2017 |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 8.00% | 8.00% | 8.00%

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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