Summary of Significant Accounting Policies - Goodwill (Details) $ in Millions	12 Months Ended
	May 31, 2019 USD ($) item	May 31, 2018 USD ($)
Goodwill by reportable segment
Number of reporting units | item	3
Goodwill, Beginning Balance	$ 118.7	$ 105.6
Acquisition		12.5
Finalization of purchase price allocation	(1.0)
Foreign currency translation adjustments	(1.5)	0.6
Goodwill, Ending Balance	$ 116.2	118.7
Aviation Services
Goodwill by reportable segment
Number of reporting units | item	2
Goodwill, Beginning Balance	$ 99.4	86.3
Acquisition		12.5
Finalization of purchase price allocation	(1.0)
Foreign currency translation adjustments	(1.5)	0.6
Goodwill, Ending Balance	$ 96.9	99.4
Expeditionary Services
Goodwill by reportable segment
Number of reporting units | item	1
Goodwill, Beginning Balance	$ 19.3	19.3
Goodwill, Ending Balance	$ 19.3	$ 19.3