Summary of All Stock Option Activity (Detail) (Stock Options, USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding at December 31, 2013	1,999,688
Granted, Number of shares	200,720	361,220	487,950
Exercised, Number of Options	(195,383)
Forfeited, Number of Options	(20,549)
Expired, Number of Options	(75,020)
Options Outstanding at December 31, 2014	1,909,456	1,999,688
Options Exercisable, Number of Options, at December 31, 2014	1,360,693
Options Outstanding at December 31, 2013	$ 11.57
Granted, Weighted Average Exercise Price	$ 13.32
Exercised, Weighted Average Exercise Price	$ 9.83
Forfeited, Weighted Average Exercise Price	$ 10.46
Expired, Weighted Average Exercise Price	$ 22.70
Options Outstanding at December 31, 2014	$ 11.79	$ 11.57
Options Exercisable,Weighted Average Exercise Price, at December 31, 2014	$ 12.01
Options Outstanding,Weighted Average Remaining Contractual Life	5 years 6 months	6 years
Options Exercisable, Weighted Average Remaining Contractual Life, at December 31, 2014	4 years 4 months 24 days
Options Outstanding at December 31, 2013	$ 4.4
Exercised, Aggregate Intrinsic Value	0.7	0.3	0.3
Options Outstanding at December 31, 2014	3.3	4.4
Options Exercisable at December 31, 2014	$ 2.5

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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 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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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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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.

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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 price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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

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