Schedule of Asset Allocation Policy and Ranges for Overall Risk and Return Objectives of Portfolio (Detail)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	100.00%	100.00%
Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	23.00%
Permitted Range, minimum	10.00%
Permitted Range, maximum	30.00%
Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	75.00%
Permitted Range, minimum	70.00%
Permitted Range, maximum	90.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	2.00%	2.00%
Permitted Range, minimum	0.00%
Permitted Range, maximum	10.00%

Defined benefit plan plan assets target allocation percentage range maximum.

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Defined benefit plan plan assets target allocation percentage range minimum.

+ References

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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 target 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/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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