Employee Benefit Plans - Aggregate Fair Value of CMRT Assets (Detail) - CMRT	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
CMRT asset value	$ 672,400
Assets not subject to fair value hierarchy	31.00%
Total percentage of CMRT assets	100.00%
Asset allocation percentage	100.00%
Equities Principally Publicly Traded \| U.S.
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	33.00%
Equities Principally Publicly Traded \| Non-US
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	25.00%
Fixed Income Securities, Principally Publically Traded
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	31.00%
Privately Managed Limited Partnerships
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	4.00%
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	5.00%
Other, Primarily Cash
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percentage	2.00%
Level 1
Defined Benefit Plan Disclosure [Line Items]
Assets subject to fair value hierarchy	54.00%
Level 2
Defined Benefit Plan Disclosure [Line Items]
Assets subject to fair value hierarchy	8.00%
Level 3
Defined Benefit Plan Disclosure [Line Items]
Assets subject to fair value hierarchy	7.00%

Combined master retirement trust asset value.

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- Definition

Percentage of combined master retirement trust assets.

+ References

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+ Details

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

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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.

+ References

No definition available.

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- Definition

Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

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