Earnings per Share - Composition of Weighted-Average Common Shares (Denominator) Used in Basic and Diluted Earnings Per Share (Detail) - shares	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Weighted Average Number of Shares Issued, Basic	113,914,902	113,914,902	113,914,902	111,100,495
Average treasury shares (in shares)	(13,550,710)	(16,436,228)	(14,783,078)	(16,465,984)
Weighted Average Number of Shares, Deferred Compensation Plans	(37,411)	0	(37,411)	0
Average unearned nonvested shares	(100,134)	(75,858)	(95,916)	(98,039)
Weighted average common shares and common stock equivalents used to calculate basic earnings per share (in shares)	100,226,647	97,402,816	98,998,497	94,536,472
Additional common stock equivalents (nonvested stock) used to calculate diluted earnings per share	226,754	54,654	161,660	42,018
Weighted Average Number Of Additional Shares Deferred Compensation Plan	(37,411)	0	(37,411)	0
Weighted average common shares and common stock equivalents used to calculate diluted earnings per share (in shares)	100,490,812	97,457,470	99,197,568	94,578,490

Weighted Average Number Of Additional Shares Deferred Compensation Plan

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Weighted Average Number of Shares, Deferred Compensation Plans

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This element represents nonvested share-based payment awards that contain non-forfeitable rights to dividends or dividend equivalents (whether paid or unpaid). Basic and dilutive earnings per share are generally disclosed for such shares using the two-class method.

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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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This element represents the weighted average total number of shares issued throughout the period including the first (beginning balance outstanding) and last (ending balance outstanding) day of the period before considering any reductions (for instance, shares held in treasury) to arrive at the weighted average number of shares outstanding. Weighted average relates to the portion of time within a reporting period that common shares have been issued and outstanding to the total time in that period. Such concept is used in determining the weighted average number of shares outstanding for purposes of calculating earnings per share (basic).

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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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Number of shares of restricted stock determined by relating the portion of time within a reporting period that restricted shares have been outstanding to the total time in that period. Restricted shares are subject to sales, contractual, regulatory or other restrictions that prevent or inhibit the holder from freely disposing of them before the restriction ends.

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Number of shares of treasury stock determined by relating the portion of time within a reporting period that treasury shares have been outstanding to the total time in that period. Treasury stock is stock that the Entity has issued but subsequently reacquired.

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