Incentive Plans - Schedule of Activity Relating to RSUs and DSUs (Detail) - $ / shares	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
RSUs [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Units, Beginning Balance	12,919,846	11,698,138
Units, Granted	3,974,389	5,134,126
Units, Forfeited	(105,853)	(92,011)
Units, Vested	(5,542,633)	(3,937,970)
Units, Ending Balance	11,245,749	12,802,283
Weighted Average Grant Date Fair Value, Beginning Balance	$ 40.23	$ 40.65
Weighted Average Grant Date Fair Value, Granted	54.07	43.07
Weighted Average Grant Date Fair Value, Forfeited	44.55	39.98
Weighted Average Grant Date Fair Value, Vested	42.86	45.36
Weighted Average Grant Date Fair Value, Ending Balance	$ 43.78	$ 40.17
DSUs [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Units, Beginning Balance	278,422	276,725
Units, Granted	36,744	38,315
Units, Vested		(43,465)
Units, Ending Balance	315,166	271,575
Weighted Average Grant Date Fair Value, Beginning Balance	$ 37.46	$ 36.05
Weighted Average Grant Date Fair Value, Granted	52.18	44.30
Weighted Average Grant Date Fair Value, Vested		35.77
Weighted Average Grant Date Fair Value, Ending Balance	$ 39.18	$ 37.26

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(3)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

+ References

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-Publisher FASB
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-SubTopic 10
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-Paragraph 2
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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

+ References

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-Publisher FASB
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- 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.

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No definition available.

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