Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018
Disaggregation of revenue by major customer markets
Net sales	$ 560.9	$ 493.3	$ 1,102.4	$ 959.6
Aviation Services
Disaggregation of revenue by major customer markets
Net sales	532.0	462.9	1,043.8	901.3
Aviation Services | North America
Disaggregation of revenue by major customer markets
Net sales	395.6	331.7	786.0	651.2
Aviation Services | Europe/Africa
Disaggregation of revenue by major customer markets
Net sales	99.8	83.5	188.0	164.5
Aviation Services | Other
Disaggregation of revenue by major customer markets
Net sales	36.6	47.7	69.8	85.6
Aviation Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	369.6	325.5	700.1	632.2
Aviation Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	162.4	137.4	343.7	269.1
Expeditionary Services
Disaggregation of revenue by major customer markets
Net sales	28.9	30.4	58.6	58.3
Expeditionary Services | North America
Disaggregation of revenue by major customer markets
Net sales	26.4	29.2	54.6	54.9
Expeditionary Services | Europe/Africa
Disaggregation of revenue by major customer markets
Net sales	2.2	1.0	3.6	2.7
Expeditionary Services | Other
Disaggregation of revenue by major customer markets
Net sales	0.3	0.2	0.4	0.7
Expeditionary Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	6.4	8.0	12.1	16.5
Expeditionary Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	$ 22.5	$ 22.4	$ 46.5	$ 41.8