Revenue recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Nov. 30, 2018	Nov. 30, 2017
Disaggregation of Revenue
Sales	$ 493.3	$ 420.6	$ 959.6	$ 818.5
Aviation Services
Disaggregation of Revenue
Sales	462.9	391.7	901.3	763.0
Aviation Services | North America
Disaggregation of Revenue
Sales	331.7	268.0	651.2	525.5
Aviation Services | Europe
Disaggregation of Revenue
Sales	83.5	86.6	164.5	155.2
Aviation Services | Other
Disaggregation of Revenue
Sales	47.7	37.1	85.6	82.3
Aviation Services | Commercial
Disaggregation of Revenue
Sales	325.5	303.4	632.2	600.2
Aviation Services | Government and defense
Disaggregation of Revenue
Sales	137.4	88.3	269.1	162.8
Expeditionary Services
Disaggregation of Revenue
Sales	30.4	28.9	58.3	55.5
Expeditionary Services | North America
Disaggregation of Revenue
Sales	29.2	27.0	54.9	51.7
Expeditionary Services | Europe
Disaggregation of Revenue
Sales	1.0	1.9	2.7	3.7
Expeditionary Services | Other
Disaggregation of Revenue
Sales	0.2		0.7	0.1
Expeditionary Services | Commercial
Disaggregation of Revenue
Sales	8.0	9.2	16.5	17.5
Expeditionary Services | Government and defense
Disaggregation of Revenue
Sales	$ 22.4	$ 19.7	$ 41.8	$ 38.0