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Schedule of Minimum and Maximum Market Value of Plan Assets (Detail)	Dec. 31, 2017
Equity Securities | Minimum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	55.00%
Equity Securities | Maximum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	75.00%
Debt Securities | Minimum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	25.00%
Debt Securities | Maximum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	45.00%
Investment Grade Bonds | Minimum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	0.00%
Investment Grade Bonds | Maximum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	20.00%
Cash and Cash Equivalents | Minimum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	0.00%
Cash and Cash Equivalents | Maximum
Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]
Cash and cash equivalents	10.00%

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- Definition

Schedule Of Pension And Other Postretirement Plan Assets By Fair Value [Line Items]

+ References

No definition available.

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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/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-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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