Stock-Based Compensation Plans (Other Information Pertaining to Restricted Stock Awards) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	May 31, 2022		May 31, 2021	May 31, 2020
Time-Based Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value per share	$ 114.21	[1]	$ 73.93	$ 84.46
Total grant-date fair value of shares vested	$ 3.8		$ 3.0	$ 3.2
Performance-Based Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value per share	$ 103.97	[1]	$ 65.17	$ 80.59
Total grant-date fair value of shares vested	$ 7.2		$ 8.4	$ 5.8

[1] For performance-based shares, granted number assumes achievement of performance goals at target. Actual number of shares to be earned may differ from this amount.