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Service Revenue (Summary of Changes in Deferred Revenue Related to Material Right Performance Obligations) (Details) - USD ($)
$ in Millions
12 Months Ended
May 31, 2020
May 31, 2019
Service Revenue [Abstract]    
Balance, beginning of period $ 45.7 $ 46.4
Deferral of revenue 24.7 27.6
Recognition of unearned revenue (27.8) (28.3)
Balance, end of period $ 42.6 $ 45.7