Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Calculation of Basic Earnings Per Share:
Net Income	$ 26,855	$ 19,144	$ 58,124
Weighted shares outstanding (shares)	16,351,338	16,317,224
Basic Earnings Per Share (in dollars per share)	$ 1.64	$ 1.17
Reconciliation of Numerator:
Net Income	$ 26,855	$ 19,144	$ 58,124
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	16,351,338	16,317,224
Effect of dilutive securities:
Share-based compensation (shares)	51,647	46,572
Adjusted denominator-Diluted (shares)	16,402,985	16,363,796
Diluted Earnings Per Share (in dollars per share)	$ 1.64	$ 1.17

Effect of dilutive securities.

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Reconciliation of denominator.

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Reconciliation of numerator:

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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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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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