Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 28, 2021	Feb. 29, 2020	Feb. 28, 2021	Feb. 29, 2020
Disaggregation of revenue by major customer markets
Net sales	$ 410.3	$ 553.1	$ 1,214.7	$ 1,655.5
Aviation Services
Disaggregation of revenue by major customer markets
Net sales	389.7	530.3	1,138.3	1,574.1
Aviation Services | North America
Disaggregation of revenue by major customer markets
Net sales	318.2	412.9	927.1	1,198.9
Aviation Services | Europe
Disaggregation of revenue by major customer markets
Net sales	49.0	85.1	145.7	273.1
Aviation Services | Other
Disaggregation of revenue by major customer markets
Net sales	22.5	32.3	65.5	102.1
Aviation Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	204.7	353.7	566.5	1,053.8
Aviation Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	185.0	176.6	571.8	520.3
Expeditionary Services
Disaggregation of revenue by major customer markets
Net sales	20.6	22.8	76.4	81.4
Expeditionary Services | North America
Disaggregation of revenue by major customer markets
Net sales	20.3	20.2	73.6	74.8
Expeditionary Services | Europe
Disaggregation of revenue by major customer markets
Net sales	0.2	2.6	2.6	6.2
Expeditionary Services | Other
Disaggregation of revenue by major customer markets
Net sales	0.1		0.2	0.4
Expeditionary Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	3.1	5.5	10.8	17.6
Expeditionary Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	$ 17.5	$ 17.3	$ 65.6	$ 63.8