Stock-Based Compensation - Additional Information (Detail) (USD $)	12 Months Ended					12 Months Ended	1 Months Ended	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010	May 31, 2012 Before June 30, 2011	May 31, 2012 After June 30, 2011	May 31, 2012 Performance Shares	Sep. 14, 2011 Market-Based Restricted Common Shares	May 31, 2012 Market-Based Restricted Common Shares Day Year	May 31, 2012 Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance	23,249,000
Stock based compensation expense, pre tax	$ 11,742,000	$ 6,173,000	$ 4,570,000
Stock based compensation expense, after tax	7,871,000	4,163,000	2,826,000
Unrecognized compensation cost	21,044,000
Unrecognized compensation cost related to non-vested awards, expense period	4 years						3 years
Purchase price percentage of fair market value on the date of grant for stock options	100.00%								100.00%
Stock options exercisable percentage per year				20.00%	33.00%
Stock options expiration period	10 years
Performance period for certain key employees						3 years
Total intrinsic value of stock options exercised	3,220,000
Cash received from the exercise of stock options	11,116,000
Tax benefit realized from the exercise of these stock options	578,000
Restricted common shares, granted								370,000
Common share awards vesting, minimum price per share								$ 30.00
Common share awards vesting, minimum consecutive days at stated price								30
Restricted common shares, fair value per share								$ 19.53
Unrecognized compensation cost							$ 5,739,000	$ 7,226,000
Derived service period								0.81
Service-based vesting condition period, years							3 years