Stock option activity (Detail) (Stock Options [Member], USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Outstanding at December 31, 2011	1,477
Shares Granted	276
Shares Exercised	(62)
Shares Outstanding at December 31, 2012	1,691
Shares Exercisable at December 31, 2012	1,202
Weighted Average Exercise Price of Shares Outstanding at December 31, 2011	$ 20.13
Weighted Average Exercise Price of Shares Granted	$ 23.35
Weighted Average Exercise Price of Shares Exercised	$ 11.37
Weighted Average Exercise Price of Shares Outstanding at December 31, 2012	$ 20.98
Weighted Average Exercise Price of Exercisable Shares at December 31, 2012	$ 19.86
Weighted Average Remaining Contractual Term of Outstanding Options at December 31, 2012	4 years 7 months
Weighted Average Remaining Contractual Term of Exercisable Option at December 31, 2012	3 years 8 months
Options Outstanding, Intrinsic Value at December 31, 2012	$ 3.1
Options, Exercisable, Intrinsic Value at December 31, 2012	$ 3.1

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

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The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options.

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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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