Stock-Based Compensation (Details 3) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
May. 31, 2015	May. 31, 2014	May. 31, 2013
Restricted stock
Restricted Stock
Compensation expense	$ 4.9	$ 4.9	$ 7.6
Number of Shares
Nonvested at the beginning of the period ( in shares)	990,000
Granted (in shares)	331,000
Vested (in shares)	(178,000)
Forfeited (in shares)	(89,000)
Nonvested at the end of the period (in shares)	1,054,000	990,000
Weighted Average Fair Value on Grant Date
Nonvested at the beginning of the period (in dollars per share)	$ 22.49
Granted (in dollars per share)	25.86
Vested (in dollars per share)	19.20
Forfeited (in dollars per share)	20.21
Nonvested at the end of the period (in dollars per share)	$ 24.38	$ 22.49
Unearned compensation not yet recognized	$ 8.5
Average remaining amortization period of unearned compensation	1 year 3 months 18 days
Performance-based restricted stock \| Executives and other key employees
Number of Shares
Granted (in shares)	188,125	60,808	53,280
Time-based restricted stock \| Board of Directors
Number of Shares
Granted (in shares)	45,000	45,000	48,333
Time-based restricted stock \| Executives and other key employees
Number of Shares
Granted (in shares)	97,581	60,808	65,780

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, 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 equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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