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Stock-Based Compensation - Shareholders' Rights Plan (Details) - Shareholders' Rights Plan
12 Months Ended
May 31, 2017
item
$ / shares
shares
Shareholders' Rights Plan  
Right to purchase number of shares | shares 1
Purchase price per share (in dollars per share) $ 140
Minimum percentage of the common stock shares acquired by a person or group of persons before rights become exercisable 15.00%
Market values of shares under the right (in dollars per share) $ 280
Market values of shares under the right, as multiplier of exercise price | item 2
Minimum percentage of assets or earning power sold before rights become exercisable 50.00%
Redemption price per Right (in dollars per share) $ 0.01