Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Anti-dilutive shares of nonvested restricted stock that were excluded from the calculation of diluted earnings per share (in shares)			24,832	23,615
Earnings per share numerator:
Net income attributable to common shareholders before allocation of earnings to participating securities	$ 58,481	$ 59,772	$ 171,931	$ 186,437
Earnings allocated to participating securities	121	118	340	370
Net income attributable to common shareholders after allocation of earnings to participating securities	$ 58,360	$ 59,654	$ 171,591	$ 186,067
Earnings per share denominator:
Weighted-average number of shares of common stock outstanding - basic and diluted (in shares)	11,409,000	11,829,000	11,606,000	11,829,000
Earnings per share - basic and diluted (in dollars per share)	$ 5.12	$ 5.04	$ 14.78	$ 15.73

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The portion of profit or loss for the period, net of income taxes, after deducting aggregate earnings distributed and earnings allocated to participating securities under the two-class method.

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or 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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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount of earnings (loss) distributed and earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

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