Benefit Plans - Status of Unexercised Stock Options (Detail) - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of stock options, outstanding at beginning of year (in shares)	470,979	507,656	703,669
Number of stock options, granted (in shares)	41,685	43,124	42,857
Number of stock options, exercised (in shares)	(13,463)	(79,801)	(238,370)
Number of stock options, forfeited (in shares)	0	0	0
Number of stock options, expired (in shares)	0	0	(500)
Number of stock options, outstanding at the end of year (in shares)	499,201	470,979	507,656
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, outstanding at beginning of year (in dollars per share)	$ 18.36	$ 16.84	$ 14.70
Weighted average exercise price, granted (in dollars per share)	27.25	25.58	26.31
Weighted average exercise price, exercised (in dollars per share)	10.35	12.61	12.22
Weighted average exercise price, forfeited (in dollars per share)	0	0	0
Weighted average exercise price, expired (in dollars per share)	0	0	17.94
Weighted average exercise price, outstanding at the end of year (in dollars per share)	$ 19.32	$ 18.36	$ 16.84

Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Net number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Number of share options (or share units) exercised during the current period.

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