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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
	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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Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Cash Deposits

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Length of natural gas transmission pipeline.

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Amount of natural gas purchase contracts outstanding under hedging program.

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Reclassification Of Short Term To Long Term Debt

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