Stock option activity (Detail) (Stock Options, USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award
Shares Exercisable	874
Weighted Average Exercise Price of Shares Outstanding at Beginning of Period	$ 21.17
Weighted Average Exercise Price of Shares Exercised	$ 23.55
Weighted Average Exercise Price of Shares Outstanding at End of Period	$ 20.23
Weighted Average Exercise Price of Exercisable Shares	$ 19.73
Weighted Average Remaining Contractual Term of Outstanding Options	3 years 292 days
Weighted Average Remaining Contractual Term of Exercisable Option	3 years 182 days
Options Outstanding, Intrinsic Value	$ 14.6
Options, Exercisable, Intrinsic Value	$ 13.2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Outstanding at Beginning of Period	1,395
Shares Exercised	(396)
Shares Outstanding at End of Period	999

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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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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 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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The number of exercisable share options (fully vested and expected to vest) that may be converted as of the balance sheet date.

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

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

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