Stock-Based Compensation Plans - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
May 31, 2012 Year	May 31, 2011	May 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration Period	10
Options exercisable, shares	1,160,420	1,040,521	905,066
Options, exercisable, weighted average price	$ 42.59	$ 41.34	$ 39.09
Shares available for future awards	335,199
Closing market price of common stock	$ 32.00
Aggregate intrinsic value of stock options outstanding	$ 5.5
Aggregate intrinsic value of exercisable stock options	3.5
Total intrinsic value of options exercised	1.0	1.7	1.5
Accrued stock-based compensation	4.1	6.8
Common stock reserved for the settlement of stock-based compensation	2,500,000	2,600,000
Liabilities increased stock-based compensation	2.6	0.8	0.5
Tax expense recognized in statements of operations for stock-based compensation	0.1	1.1	1.0
Tax benefit recognized in statements of operations for stock-based compensation	0	0
Cash tax benefit realized on stock-based compensation			0.7
Total unrecognized compensation cost related to stock options, restricted stock payments, and stock awards expected to be recognized	11.9
Unrecognized compensation cost related to stock options expected to be recognized, 2013	4.2
Unrecognized compensation cost related to stock options expected to be recognized, 2014	3.4
Unrecognized compensation cost related to stock options expected to be recognized, 2015	2.3
Unrecognized compensation cost related to stock options expected to be recognized, 2016	1.5
Unrecognized compensation cost related to stock options expected to be recognized, 2017	0.5
Selling, General and Administrative Expenses
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation included in selling, general and administrative expense	$ 2.4	$ 5.6	$ 5.1
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, outstanding	133,315
Deferred Compensation Agreements To Be Settled In Cash
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, outstanding	101,790
Deferred Compensation Agreements To Be Settled In Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, outstanding	2,995
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	18,334

Closing market price of common stock

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The total recognized tax expense related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Employee service share based compensation unrecognized compensation cost expected to be recognized in year five.

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Employee service share based compensation unrecognized compensation cost expected to be recognized in year four.

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Employee service share based compensation unrecognized compensation cost expected to be recognized in year one.

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Employee service share based compensation unrecognized compensation cost expected to be recognized in year three.

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Employee service share based compensation unrecognized compensation cost expected to be recognized in year two.

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Increase In Liabilities For Stock Based Compensation As Result Of Changes In Stock Price

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Stock Option Grants Expiration Date

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Aggregate number of common shares reserved for future issuance.

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Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements payable after one year (or the operating cycle, if longer).

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As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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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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The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of fully vested and expected to vest options that are exercisable.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise prices of fully vested and expected to vest options outstanding.

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As of the balance sheet date, the number of shares into which fully vested and expected to vest stock options outstanding can be converted under the option plan.

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