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Environmental Matters - Additional Information (Detail) - Environmental Restoration Costs $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Maine
Site Contingency [Line Items]
Amortization period for environmental costs	5 years
New Hampshire
Site Contingency [Line Items]
Amortization period for environmental costs	7 years
Northern Utilities Manufactured Gas Plant Sites
Site Contingency [Line Items]
Estimated Costs Accrued For Remediation	$ 0.8

X

- Definition

The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109237650&loc=d3e13064-110858

Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13296-110859

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