Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Feb. 28, 2019	Feb. 29, 2020	Feb. 28, 2019
Disaggregation of revenue by major customer markets
Net sales	$ 553.1	$ 529.5	$ 1,655.5	$ 1,489.1
Aviation Services
Disaggregation of revenue by major customer markets
Net sales	530.3	497.3	1,574.1	1,398.6
Aviation Services | North America
Disaggregation of revenue by major customer markets
Net sales	412.9	373.7	1,198.9	1,024.9
Aviation Services | Europe/Africa
Disaggregation of revenue by major customer markets
Net sales	85.1	80.9	273.1	245.4
Aviation Services | Other
Disaggregation of revenue by major customer markets
Net sales	32.3	42.7	102.1	128.3
Aviation Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	353.7	350.8	1,053.8	983.0
Aviation Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	176.6	146.5	520.3	415.6
Expeditionary Services
Disaggregation of revenue by major customer markets
Net sales	22.8	32.2	81.4	90.5
Expeditionary Services | North America
Disaggregation of revenue by major customer markets
Net sales	20.2	30.2	74.8	85.1
Expeditionary Services | Europe/Africa
Disaggregation of revenue by major customer markets
Net sales	2.6	1.5	6.2	4.2
Expeditionary Services | Other
Disaggregation of revenue by major customer markets
Net sales		0.5	0.4	1.2
Expeditionary Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	5.5	6.8	17.6	23.3
Expeditionary Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	$ 17.3	$ 25.4	$ 63.8	$ 67.2