Summary of Significant Accounting Policies - Goodwill (Details) $ in Millions	12 Months Ended
May 31, 2021 USD ($) item	May 31, 2020 USD ($)
Goodwill by reportable segment
Number of reporting units \| item	3
Goodwill impairment charges	$ 0.0	$ 0.0
Goodwill, Beginning Balance	115.7	116.2
Sale of Composites	(0.5)
Foreign currency translation adjustments	4.1	(0.5)
Goodwill, Ending Balance	$ 119.3	115.7
Aviation Services
Goodwill by reportable segment
Number of reporting units \| item	2
Goodwill, Beginning Balance	$ 96.4	96.9
Foreign currency translation adjustments	4.1	(0.5)
Goodwill, Ending Balance	$ 100.5	96.4
Expeditionary Services
Goodwill by reportable segment
Number of reporting units \| item	1
Goodwill, Beginning Balance	$ 19.3	19.3
Sale of Composites	(0.5)
Goodwill, Ending Balance	$ 18.8	$ 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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