Summary of Significant Accounting Policies - Goodwill (Details) $ in Millions	12 Months Ended
	May 31, 2018 USD ($) item	May 31, 2017 USD ($)
Goodwill by reportable segment
Total number of reporting units | item	5
Number of reporting units assigned for goodwill | item	4
Goodwill, Beginning Balance	$ 105.6	$ 107.5
Acquisition	12.5
Foreign currency translation adjustments	0.6	(1.9)
Goodwill, Ending Balance	$ 118.7	105.6
Aviation Services
Goodwill by reportable segment
Number of reporting units assigned for goodwill | item	2
Goodwill, Beginning Balance	$ 64.4	66.3
Acquisition	12.5
Foreign currency translation adjustments	0.6	(1.9)
Goodwill, Ending Balance	$ 77.5	64.4
Expeditionary Services
Goodwill by reportable segment
Number of reporting units assigned for goodwill | item	2
Goodwill, Beginning Balance	$ 41.2	41.2
Goodwill, Ending Balance	$ 41.2	$ 41.2