Summary of Significant Accounting Policies - Goodwill (Details) $ in Millions	12 Months Ended
May 31, 2020 USD ($) item	May 31, 2019 USD ($)
Goodwill by reportable segment
Number of reporting units \| item	3
Goodwill impairment charges	$ 0.0
Goodwill, Beginning Balance	116.2	$ 118.7
Finalization of purchase price allocation		(1.0)
Foreign currency translation adjustments	(0.5)	(1.5)
Goodwill, Ending Balance	$ 115.7	116.2
Aviation Services
Goodwill by reportable segment
Number of reporting units \| item	2
Goodwill, Beginning Balance	$ 96.9	99.4
Finalization of purchase price allocation		(1.0)
Foreign currency translation adjustments	(0.5)	(1.5)
Goodwill, Ending Balance	$ 96.4	96.9
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

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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