Business Segment Information - Customers Percentage of Sales (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2021	May 31, 2020	May 31, 2019
Business Segment Information
Net sales	$ 1,652.3	$ 2,072.0	$ 2,051.8
U.S. Department of Defense, other U.S. government agencies and their contractors
Business Segment Information
Net sales	738.8	668.2	546.2
Aviation Services
Business Segment Information
Net sales	1,553.7	1,964.2	1,920.6
Aviation Services | U.S. Department of Defense, other U.S. government agencies and their contractors
Business Segment Information
Net sales	657.0	588.7	455.9
Expeditionary Services
Business Segment Information
Net sales	98.6	107.8	131.2
Expeditionary Services | U.S. Department of Defense, other U.S. government agencies and their contractors
Business Segment Information
Net sales	$ 81.8	$ 79.5	$ 90.3
Customer | Total sales | U.S. Department of Defense, other U.S. government agencies and their contractors
Business Segment Information
Percentage of total sales	44.70%	32.20%	26.60%