Other Comprehensive Income (Loss) - Amounts Reclassified Out of Each Component of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (loss) on sale of securities	$ (12)	$ (19)	$ 18
Income taxes	6,360	6,619	4,781
Net of tax	(14,628)	(15,716)	(11,168)
Reclassifications, net of tax	(239)	(202)	(189)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	8	13	(12)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member] \| Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (loss) on sale of securities	12	19	(18)
Income taxes	(4)	(6)	6
Net of tax	(8)	(13)	12
Accumulated Defined Benefit Plans Adjustment, Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, wages and benefits	(380)	(326)	(270)
Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	133	111	93
Reclassifications, net of tax	$ (247)	$ (215)	$ (177)

This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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