Stock Option Plan - Summary of Option Activity (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shares
Outstanding at January 1 (in shares)	753,625	570,840	674,585
Granted (in shares)	200,000	237,500	277,500
Exercised (in shares)	(167,917)	(49,715)	(149,995)
Expired/Forfeited (in shares)	(37,500)	(5,000)	(231,250)
Outstanding December 31 (in shares)	748,208	753,625	570,840
Exercisable at December 31 (in shares)	380,708	413,000	377,715
Weighted Average Exercise Price
Outstanding at January 1 (in usd per share)	$ 42.55	$ 41.04	$ 40.40
Granted (in usd per share)	$ 47.03	$ 44.42	$ 39.29
Exercised (in usd per share)	$ 37.71	$ 33.15	$ 31.03
Expired/Forfeited (in usd per share)	$ 43.56	$ 52.16	$ 43.56
Outstanding December 31 (in usd per share)	$ 44.79	$ 42.55	$ 41.04
Exercisable at December 31 (in usd per share)	$ 44.85	$ 42.42	$ 41.41
Intrinsic value of options exercised	$ 2.0	$ 0.6	$ 1.6
Intrinsic value of options outstanding	9.3
Intrinsic value of options exercisable	$ 4.7
Closing price (in usd per share)	$ 57.19us-gaap_SharePrice
Weighted average remaining life of options exercisable	5 years 5 months 8 days
Weighted average remaining life of options outstanding	6 years 11 months 15 days

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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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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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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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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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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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 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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Price of a single share of a number of saleable stocks of a company.

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

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