Business Segment Information - Sales and Gross Profit (Details) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2021 USD ($)	Nov. 30, 2020 USD ($)	Nov. 30, 2021 USD ($) segment	Nov. 30, 2020 USD ($)
Disaggregation of revenue by major customer markets
Operating segments (in segments) | segment			2
Sales	$ 436.6	$ 403.6	$ 891.7	$ 804.4
Gross profit	78.4	69.5	143.0	118.1
Aviation Services
Disaggregation of revenue by major customer markets
Sales	419.3	385.0	854.9	748.6
Gross profit	74.0	66.8	134.9	111.4
Expeditionary Services
Disaggregation of revenue by major customer markets
Sales	17.3	18.6	36.8	55.8
Gross profit	$ 4.4	$ 2.7	$ 8.1	$ 6.7