Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Calculation of Basic Earnings Per Share:
Net Income	$ 28,664	$ 26,855
Weighted shares outstanding (shares)	16,384,927	16,351,338
Basic Earnings Per Share (in dollars per share)	$ 1.75	$ 1.64
Reconciliation of Numerator:
Net Income	$ 28,664	$ 26,855
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	16,384,927	16,351,338
Effect of dilutive securities:
Share-based compensation (shares)	47,925	51,647
Adjusted denominator-Diluted (shares)	16,432,852	16,402,985
Diluted Earnings Per Share (in dollars per share)	$ 1.74	$ 1.64

Effect of dilutive securities.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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