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Retirement Plans - Schedule of Target Allocation and Expected Long-Term Rate of Return by Asset Category (Detail)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%	100.00%
Equity Securities in Financial Institutions [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	48.00%	47.50%
Equity Securities in Financial Institutions [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	20.00%
Equity Securities in Financial Institutions [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	50.00%
Total Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	51.90%	52.10%
Total Debt Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	30.00%
Total Debt Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	60.00%
Money Market Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	0.10%	0.40%
Money Market Funds [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	20.00%
Money Market Funds [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	30.00%

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- Definition

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+ References

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- Definition

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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