Accounting for Stock-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				1 Months Ended
	Aug. 31, 2014 Stock options	Aug. 31, 2013 Stock options	Aug. 31, 2014 Restricted stock	Aug. 31, 2013 Restricted stock	Jul. 31, 2014 Performance-based restricted stock	Jul. 31, 2014 Time-based restricted stock	Jun. 30, 2014 Time-based restricted stock Board of Directors
Stock options, additional disclosures
Granted (in shares)					192,250	95,956	45,000
Granted (in dollars per share)					$ 26.08	$ 26.08	$ 24.50
Total intrinsic value of stock options exercised	$ 1.0	$ 2.6
Compensation expense	$ 0.7	$ 0.5	$ 1.3	$ 1.1
Stock options granted (in shares)	0