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Derivatives, Hedges, Financial Instruments and Carbon Credits - Schedule of Carrying Values and Estimated Fair Values of Financial Instruments (Parenthetical) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Notes based on quoted price	$ 99.25	$ 94.88

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- References

No definition available.

+ Details

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- Definition

Price at which a dealer is willing to buy an instrument, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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