Stock-Based Compensation (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	May 31, 2014	May 31, 2013	May 31, 2012
Restricted stock
Restricted Stock
Compensation expense	$ 4.9	$ 7.6	$ 8.4
Number of Shares
Nonvested at the beginning of the period ( in shares)	1,253,000
Granted (in shares)	167,000
Vested (in shares)	(400,000)
Forfeited (in shares)	(30,000)
Nonvested at the end of the period (in shares)	990,000	1,253,000
Weighted Average Fair Value on Grant Date
Nonvested at the beginning of the period (in dollars per share)	$ 22.91
Granted (in dollars per share)	$ 24.15
Vested (in dollars per share)	$ 24.36
Forfeited (in dollars per share)	$ 27.65
Nonvested at the end of the period (in dollars per share)	$ 22.49	$ 22.91
Unearned compensation not yet recognized	$ 6.5
Average remaining amortization period of unearned compensation	1 year 4 months 24 days
Performance-based restricted stock | Executives and other key employees
Number of Shares
Granted (in shares)	60,808	53,280	362,500
Time-based restricted stock | Board of Directors
Number of Shares
Granted (in shares)	45,000	48,333	45,000
Time-based restricted stock | Executives and other key employees
Number of Shares
Granted (in shares)	60,808	65,780	299,491