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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  | 9 Months Ended |  |  |  |  |  |  | 9 Months Ended | 0 Months Ended |
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Sep. 30, 2014 mi Bcf | Sep. 30, 2013 Bcf | Dec. 31, 2013 Bcf | Sep. 30, 2014 Maximum | Sep. 30, 2014 Customer information system | Sep. 30, 2014 ISO-NE Obligations | Dec. 31, 2013 ISO-NE Obligations | Sep. 30, 2013 ISO-NE Obligations | Sep. 30, 2014 Natural Gas Hedging Program | Dec. 31, 2013 Natural Gas Hedging Program | Sep. 30, 2013 Natural Gas Hedging Program | Sep. 30, 2014 Utilities Subsidiary | Sep. 30, 2014 Other Subsidiaries Subsidiary | Oct. 15, 2014 Northern Utilities Inc Subsequent Event | Oct. 15, 2014 Northern Utilities Inc 4.42% Senior Unsecured Notes due October 15, 2044 Subsequent Event | Oct. 15, 2014 Northern Utilities Inc 4.42% Senior Unsecured Notes due October 15, 2044 Subsequent Event
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Subsidiaries |  |  |  |  |  |  |  |  |  |  |  | 3 | 3 |  |  |
Length Of Pipeline | 86 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash Deposits |  |  |  |  |  | $ 8.0 | $ 7.3 | $ 8.8 | $ 0 | $ 0 | $ 0.7 |  |  |  |  |
Cash equivalents maturity period |  |  |  | 3 months |  |  |  |  |  |  |  |  |  |  |  |
Cost of removal obligation | 63.2 | 56.7 | 57.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Construction Work in Progress | 54.2 | 58.2 | 24.6 |  | 6.9 |  |  |  |  |  |  |  |  |  |  |
Number of Natural Gas Storage Outstanding | 3.0 | 2.6 | 1.8 |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50
Long-term debt, stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.42%
Long-term debt, maturity date |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Oct. 15, 2044 |
Repayment of short-term debt | 16.5 | 5.8 |  |  |  |  |  |  |  |  |  |  |  | 42.3 |  |
Reclassification of short-term debt to long-term debt | $ 42.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Details
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