Revenue Recognition - Revenue by Product Class and Timing (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2025	Nov. 30, 2024	Nov. 30, 2025	Nov. 30, 2024
Disaggregation Of Revenue [Line Items]
Net sales	$ 871.9	$ 739.0	$ 1,744.8	$ 1,573.0
Direct | Steel Processing
Disaggregation Of Revenue [Line Items]
Net sales	844.1	700.9	1,683.6	1,490.8
Toll | Steel Processing
Disaggregation Of Revenue [Line Items]
Net sales	$ 27.8	$ 38.1	$ 61.2	$ 82.2