Business Segment Information - Sales and Gross Profit (Details) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 28, 2021 USD ($)	Feb. 29, 2020 USD ($)	Feb. 28, 2021 USD ($) segment	Feb. 29, 2020 USD ($)
Disaggregation of revenue by major customer markets
Operating segments (in segments) | segment			2
Sales	$ 410.3	$ 553.1	$ 1,214.7	$ 1,655.5
Gross profit	86.0	65.3	204.1	232.8
Aviation Services
Disaggregation of revenue by major customer markets
Sales	389.7	530.3	1,138.3	1,574.1
Gross profit	82.5	65.2	193.9	230.9
Expeditionary Services
Disaggregation of revenue by major customer markets
Sales	20.6	22.8	76.4	81.4
Gross profit	$ 3.5	$ 0.1	$ 10.2	$ 1.9