Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2018	Aug. 31, 2017
Disaggregation of Revenue
Sales	$ 466.3	$ 397.9
Aviation Services
Disaggregation of Revenue
Sales	438.4	371.3
Aviation Services | North America
Disaggregation of Revenue
Sales	319.5	257.5
Aviation Services | Europe/Africa
Disaggregation of Revenue
Sales	81.0	68.6
Aviation Services | Other
Disaggregation of Revenue
Sales	37.9	45.2
Aviation Services | Commercial
Disaggregation of Revenue
Sales	306.7	296.8
Aviation Services | Government and defense
Disaggregation of Revenue
Sales	131.7	74.5
Expeditionary Services
Disaggregation of Revenue
Sales	27.9	26.6
Expeditionary Services | North America
Disaggregation of Revenue
Sales	25.7	24.7
Expeditionary Services | Europe/Africa
Disaggregation of Revenue
Sales	1.7	1.8
Expeditionary Services | Other
Disaggregation of Revenue
Sales	0.5	0.1
Expeditionary Services | Commercial
Disaggregation of Revenue
Sales	8.5	8.3
Expeditionary Services | Government and defense
Disaggregation of Revenue
Sales	$ 19.4	$ 18.3