Summary of Significant Accounting Policies - Earnings per share (Details)	12 Months Ended
	May 31, 2014	May 31, 2013	May 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Basic weighted average shares outstanding	72,119,000	77,767,000	78,829,000
Plus: Dilutive effect of stock options and other share-based awards	569,000	460,000	602,000
Diluted weighted average shares outstanding	72,688,000	78,227,000	79,431,000
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted share base		300,000	200,000