Stock-Based Compensation - Additional Information (Detail) - USD ($)	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance	5,039,014
Stock based compensation expense, pre tax	$ 17,916,000	$ 22,017,000	$ 13,270,000
Stock based compensation expense, after tax	11,500,000	$ 13,778,000	$ 8,339,000
Unrecognized compensation cost	$ 16,367,000
Unrecognized compensation cost related to non-vested awards, expense period	3 years
Purchase price percentage of fair market value on the date of grant for stock options	100.00%
Stock options expiration period	10 years
Total intrinsic value of stock options exercised	$ 13,218,000
Cash received from the exercise of stock options	2,015,000
Tax benefit realized from the exercise of these stock options	$ 7,178,000
Service-based restricted common shares granted to employees cliff vesting period	3 years
Before June 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting percentage	20.00%
After June 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting percentage	33.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price percentage of fair market value on the date of grant for stock options	100.00%
Market-Based Restricted Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted common shares, granted	50,000	360,000
Common share awards vesting, minimum price per share	$ 60.00	$ 50.00
Common share awards vesting, minimum consecutive days at stated price	30 days	30 days
Restricted common shares, fair value per share	$ 32.06	$ 24.19
Unrecognized compensation cost	$ 1,603,000	$ 8,708,000
Service-based vesting condition period, years	5 years