Business Segment Information - Sales and Gross Profit (Details) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2020 USD ($)	Nov. 30, 2019 USD ($)	Nov. 30, 2020 USD ($) segment	Nov. 30, 2019 USD ($)
Disaggregation of revenue by major customer markets
Operating segments (in segments) | segment			2
Sales	$ 403.6	$ 560.9	$ 804.4	$ 1,102.4
Gross profit	69.5	85.9	118.1	167.5
Aviation Services
Disaggregation of revenue by major customer markets
Sales	385.0	532.0	748.6	1,043.8
Gross profit	66.8	85.7	111.4	165.7
Expeditionary Services
Disaggregation of revenue by major customer markets
Sales	18.6	28.9	55.8	58.6
Gross profit	$ 2.7	$ 0.2	$ 6.7	$ 1.8