Stock Incentive Plans and Stock Purchase Plans - Schedule of Activity and Certain Other Information Related to Stock Option Awards (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Shares, Outstanding at January 1, 2017	410,286
Number of Shares, Exercised	(163,944)
Number of Shares, Outstanding at December 31, 2017	246,342	410,286
Number of Shares, Exercisable at December 31, 2017	246,342
Weighted Average Exercise Price, Outstanding at January 1, 2017	$ 28.33
Weighted Average Exercise Price, Exercised	29.57
Weighted Average Exercise Price, Outstanding at December 31, 2017	27.51	$ 28.33
Weighted Average Exercise Price, Exercisable at December 31, 2017	$ 27.51
Weighted Average Remaining Contractual Term (in years), Outstanding at December 31, 2017	1 year 6 months
Weighted Average Remaining Contractual Term (in years), Exercisable at December 31, 2017	1 year 6 months
Aggregate Intrinsic Value, Exercised	$ 5.5	$ 6.8	$ 11.3
Aggregate Intrinsic Value, Outstanding at December 31, 2017	6.4
Aggregate Intrinsic Value, Exercisable at December 31, 2017	$ 6.4

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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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-Publisher FASB
-Name Accounting Standards Codification
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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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

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

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