Summary of Significant Accounting Policies - Goodwill (Details) $ in Millions	12 Months Ended
	May 31, 2017 USD ($) item	May 31, 2016 USD ($)
Goodwill by reportable segment
Total number of reporting units | item	5
Number of reporting units assigned for goodwill | item	4
Goodwill, Beginning Balance	$ 117.3	$ 123.5
Acquisition		1.3
Deferred tax adjustments		(6.5)
Foreign currency translation adjustments	(1.9)	(1.0)
Goodwill, Ending Balance	$ 115.4	117.3
Aviation Services
Goodwill by reportable segment
Number of reporting units assigned for goodwill | item	2
Goodwill, Beginning Balance	$ 66.3	73.8
Deferred tax adjustments		(6.5)
Foreign currency translation adjustments	(1.9)	(1.0)
Goodwill, Ending Balance	$ 64.4	66.3
Expeditionary Services
Goodwill by reportable segment
Number of reporting units assigned for goodwill | item	2
Goodwill, Beginning Balance	$ 51.0	49.7
Acquisition		1.3
Goodwill, Ending Balance	$ 51.0	$ 51.0