Other Comprehensive Income - Amounts Reclassified Out of Each Component of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (loss) on sale of securities	$ 9	$ (18)	$ 9	$ (20)
Income taxes	1,318	1,304	4,534	5,251
Net of tax	(3,352)	(3,306)	(10,957)	(12,430)
Reclassifications, net of tax	(288)	(43)	(631)	(152)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	(6)	12	(6)	13
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	(9)	18	(9)	20
Income taxes	3	(6)	3	(7)
Net of tax	(6)	12	(6)	13
Accumulated Defined Benefit Plans Adjustment, Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, wages and benefits	(426)	(83)	(946)	(249)
Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	144	28	321	84
Reclassifications, net of tax	$ (282)	$ (55)	$ (625)	$ (165)

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
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-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 9
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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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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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