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Environmental Matters - Additional Information (Detail) - Environmental Restoration Costs	3 Months Ended
	Mar. 31, 2018
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

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Remaining Amortization Period.

+ References

No definition available.

+ Details

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Period Type:	duration

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- Details

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Namespace Prefix:
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Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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