Stock Compensation Plans - Stock Option Transactions Under Various Stock-Based Employee Compensation Plans (Detail) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding, Beginning Balance, Shares	1,099,877
Granted, Shares	104,000
Exercised, Shares	(361,618)	(554,049)	(470,029)
Forfeited, Shares	(5,400)
Outstanding, Ending Balance, Shares	836,859	1,099,877
Exercisable, Ending Balance, Shares	650,359
Outstanding, Beginning Balance, Weighted Average Exercise Price	$ 44.00
Granted, Weighted Average Exercise Price	67.21
Exercised, Weighted-Average Exercise Price	37.15
Forfeited, Weighted Average Exercise Price	73.56
Outstanding, Ending Balance, Weighted Average Exercise Price	49.65	$ 44.00
Exercisable, Ending Balance, Weighted Average Exercise Price	$ 44.59
Outstanding, Ending balance, Weighted Average Remaining Contractual Term	5 years 4 months 2 days
Exercisable, Ending Balance, Weighted Average Remaining Contractual Term	4 years 5 months 5 days

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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