Earnings Per Common Share - Summary of Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income	$ 84,242	$ 86,135	$ 81,012
Denominator:
Weighted-average common shares outstanding	15,295,573	15,332,553	15,364,281
Effect of dilutive PBRSUs	94,779	72,607	40,459
Earnings per common share:	15,390,352	15,405,160	15,404,740
Earnings per common share:
Basic earnings per common share	$ 5.51	$ 5.62	$ 5.27
Diluted earnings per common share	$ 5.47	$ 5.59	$ 5.26

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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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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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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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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