Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2022	Aug. 31, 2021
Disaggregation of revenue by major customer markets
Net sales	$ 446.3	$ 455.1
Aviation Services
Disaggregation of revenue by major customer markets
Net sales	424.0	435.6
Aviation Services | North America
Disaggregation of revenue by major customer markets
Net sales	323.3	347.6
Aviation Services | Europe/Africa
Disaggregation of revenue by major customer markets
Net sales	62.4	62.0
Aviation Services | Other.
Disaggregation of revenue by major customer markets
Net sales	38.3	26.0
Aviation Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	292.1	267.2
Aviation Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	131.9	168.4
Expeditionary Services
Disaggregation of revenue by major customer markets
Net sales	22.3	19.5
Expeditionary Services | North America
Disaggregation of revenue by major customer markets
Net sales	21.9	19.3
Expeditionary Services | Europe/Africa
Disaggregation of revenue by major customer markets
Net sales	0.4	0.2
Expeditionary Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	1.5	0.1
Expeditionary Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	$ 20.8	$ 19.4