Share-Based Awards and Options - Restricted Stock and Employee Stock Purchase Plan (Details) (USD $)	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010
Restricted Stock
Share-based compensation cost	$ 16,391,000	$ 15,885,000	$ 18,072,000
Employee Stock Purchase Plan
Shares issued in period (in shares)			1,584,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Vesting percentage	25.00%
Restricted Stock
Nonvested, beginning of period (in shares)	869,000	713,000
Nonvested, Weighted Average Grant-Date Fair Value, beginning of period (in dollars per share)	$ 40	$ 42
Granted (in shares)	472,000	461,000
Granted, Weighted Average Grant-Date Fair Value (in dollars per share)	$ 48	$ 38
Vested (in shares)	(321,000)	(259,000)
Vested, Weighted Average Grant-Date Fair Value (in dollars per share)	$ 40	$ 42
Forfeited (in shares)	(79,000)	(46,000)
Forfeited, Weighted Average Grant-Date Fair Value (in dollars per share)	$ 43	$ 40
Nonvested, end of period (in shares)	941,000	869,000	713,000
Nonvested, Weighted Average Grant-Date Fair Value, end of period (in dollars per share)	$ 44	$ 40	$ 42
Total fair value of share awards vested in period	12,900,000	10,800,000	12,400,000
Share-based compensation cost	13,600,000	12,500,000	12,100,000
Total unrecognized compensation cost	29,300,000
Total unrecognized compensation cost, weighted average period (in years)	2.0
PRSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Vesting period (in years)	4 years
TSR [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Vesting period (in years)	3 years
Employee Stock Purchase Plan [Member]
Restricted Stock
Share-based compensation cost	500,000	500,000	500,000
Employee Stock Purchase Plan
Common stock authorized (in shares)	2,400,000
Maximum employee subscription amount	$ 25,000
Maximum employee subscription rate	20.00%
Discounted market value	85.00%
Shares issued in period (in shares)	1,000,000
Shares reserved for future issuance (in shares)	1,400,000
Weighted average grant-date fair value	$ 7	$ 6	$ 7
Discount from market price	15.00%