Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2021	Aug. 31, 2020
Disaggregation of revenue by major customer markets
Net sales	$ 455.1	$ 400.8
Aviation Services
Disaggregation of revenue by major customer markets
Net sales	435.6	363.6
Aviation Services | North America
Disaggregation of revenue by major customer markets
Net sales	347.6	294.8
Aviation Services | Europe
Disaggregation of revenue by major customer markets
Net sales	62.0	51.0
Aviation Services | Other
Disaggregation of revenue by major customer markets
Net sales	26.0	17.8
Aviation Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	267.2	169.6
Aviation Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	168.4	194.0
Expeditionary Services
Disaggregation of revenue by major customer markets
Net sales	19.5	37.2
Expeditionary Services | North America
Disaggregation of revenue by major customer markets
Net sales	19.3	34.9
Expeditionary Services | Europe
Disaggregation of revenue by major customer markets
Net sales	0.2	2.2
Expeditionary Services | Other
Disaggregation of revenue by major customer markets
Net sales	0.0	0.1
Expeditionary Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	0.1	5.7
Expeditionary Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	$ 19.4	$ 31.5