Employee Benefit Plans - Aggregate Fair Value of CMRT Assets (Detail) - CMRT - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
CMRT asset value	$ 672.4	$ 637.8
Assets not subject to fair value hierarchy	31.00%	30.00%
Total percentage of CMRT assets	100.00%	100.00%
Asset allocation percentage	100.00%	100.00%
Equities Principally Publicly Traded \| U.S.
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	33.00%	31.00%
Equities Principally Publicly Traded \| Non-US
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	25.00%	22.00%
Fixed Income Securities, Principally Publically Traded
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	31.00%	36.00%
Privately Managed Limited Partnerships
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	4.00%	5.00%
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	5.00%	5.00%
Other, Primarily Cash
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	2.00%	1.00%
Level 1
Defined Benefit Plan Disclosure [Line Items]
Assets subject to fair value hierarchy	54.00%	54.00%
Level 2
Defined Benefit Plan Disclosure [Line Items]
Assets subject to fair value hierarchy	8.00%	8.00%
Level 3
Defined Benefit Plan Disclosure [Line Items]
Assets subject to fair value hierarchy	7.00%	8.00%

Combined master retirement trust asset value.

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Percentage of combined master retirement trust assets.

+ References

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- Definition

Percentage of combined master retirement trust assets not subject to fair value hierarchy.

+ References

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Percentage of combined master retirement trust assets subject to fair value hierarchy.

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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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Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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