Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2020	Nov. 30, 2019	Nov. 30, 2020	Nov. 30, 2019
Disaggregation of revenue by major customer markets
Net sales	$ 403.6	$ 560.9	$ 804.4	$ 1,102.4
Aviation Services
Disaggregation of revenue by major customer markets
Net sales	385.0	532.0	748.6	1,043.8
Aviation Services | North America
Disaggregation of revenue by major customer markets
Net sales	314.1	395.6	608.9	786.0
Aviation Services | Europe
Disaggregation of revenue by major customer markets
Net sales	45.7	99.8	96.7	188.0
Aviation Services | Other
Disaggregation of revenue by major customer markets
Net sales	25.2	36.6	43.0	69.8
Aviation Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	192.2	369.6	361.8	700.1
Aviation Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	192.8	162.4	386.8	343.7
Expeditionary Services
Disaggregation of revenue by major customer markets
Net sales	18.6	28.9	55.8	58.6
Expeditionary Services | North America
Disaggregation of revenue by major customer markets
Net sales	18.4	26.4	53.3	54.6
Expeditionary Services | Europe
Disaggregation of revenue by major customer markets
Net sales	0.2	2.2	2.4	3.6
Expeditionary Services | Other
Disaggregation of revenue by major customer markets
Net sales		0.3	0.1	0.4
Expeditionary Services | Commercial
Disaggregation of revenue by major customer markets
Net sales	2.0	6.4	7.7	12.1
Expeditionary Services | Government and defense
Disaggregation of revenue by major customer markets
Net sales	$ 16.6	$ 22.5	$ 48.1	$ 46.5