Benefit Plans - Summary of Company's Option Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Beginning Balance	1,584,435	1,410,650	1,343,760
Number of Options, granted	299,600	881,800	456,200
Number of Options, exercised	(398,275)	(368,085)	(270,650)
Number of Options, cancelled/forfeited/expired	(276,900)	(339,930)	(118,660)
Number of Options, Ending Balance	1,208,860	1,584,435	1,410,650
Number of Options, Exercisable at December 31, 2017	325,680
Number of Options, Expected to Vest after December 31, 2017	680,049
Weighted-Average Price, Beginning Balance	$ 28.14	$ 29.91	$ 27.25
Weighted-Average Price, Options granted	34.66	25.26	31.69
Weighted-Average Price, Options exercised	28.94	27.34	19.81
Weighted-Average Price, Options cancelled/forfeited/expired	28.04	28.62	29.58
Weighted-Average Price, Ending Balance	29.52	$ 28.14	$ 29.91
Weighted-Average Price, Exercisable at December 31, 2017	28.65
Weighted-Average Price, Expected to Vest after December 31, 2017	$ 29.78
Weighted-Average Remaining Contractual Term, Balance At December 31, 2017	4 years 11 months 8 days
Weighted-Average Remaining Contractual Term, Exercisable at December 31, 2017	4 years 14 days
Weighted-Average Remaining Contractual Term, Expected to Vest after December 31, 2017	5 years 3 months 10 days
Aggregate Intrinsic Value, Balance At December 31, 2017	$ 21.1
Aggregate Intrinsic Value, Exercisable at December 31, 2017	6.0
Aggregate Intrinsic Value, Expected to Vest after December 31, 2017	$ 11.7

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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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For presentations that combine terminations, 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 or that expired.

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Weighted average price of options that were either forfeited or expired.

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

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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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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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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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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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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