Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
May 31, 2014	May 31, 2013	May 31, 2012
Schedule summarizes stock option activity
Shares Outstanding at beginning of year	2,768,928	2,985,192	2,680,390
Shares Granted	391,175	406,700	1,434,000
Shares Exercised	(105,676)	(16,835)	(193,684)
Shares Forfeited	(278,646)	(589,787)	(917,126)
Shares Expired	(102,030)	(16,342)	(18,388)
Shares Outstanding at end of year	2,673,751	2,768,928	2,985,192
Shares Options exercisable at year end	1,675,790	1,601,028	1,678,559
Shares Option expected to vest in future periods	845,256	1,069,119	1,075,473
Weighted Average exercise price, Outstanding at beginning of year	$ 14.84	$ 15.69	$ 15.96
Weighted Average exercise price Granted	$ 13.01	$ 11.40	$ 13.70
Weighted Average exercise price Exercised	$ 15.38	$ 11.15	$ 14.22
Weighted Average exercise price Forfeited	$ 17.45	$ 17.45	$ 14.22
Weighted Average exercise price Expired	$ 15.69	$ 15.69	$ 24.44
Weighted Average exercise price, Outstanding at end of year	$ 14.82	$ 14.84	$ 15.69
Weighted Average exercise price, Options exercisable at year end	$ 16.12	$ 16.12	$ 17.01
Weighted Average exercise price, Options expected to vest in future periods	$ 13.31	$ 13.31	$ 14.19
Weighted Average remaining contractual life	4 years 6 months 22 days
Weighted Average remaining contractual life, Option exercisable at year end	4 years 6 months 16 days
Weighted Average remaining contractual life, Option expected to vest in future periods	5 years 22 days
Aggregate intrinsic value, Outstanding at end of year	$ 17,540
Aggregate intrinsic value, Options exercisable at year end	12,906
Aggregate intrinsic value, Option expected to vest in future periods	$ 3,986

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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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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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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Amount by which the current fair value of the underlying stock exceeds the exercise price 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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As of the balance sheet date, the weighted-average exercise price for outstanding stock options that are fully vested or expected to vest.

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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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