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Service Revenue (Summary of Changes in Deferred Revenue Related to Material Right Performance Obligations) (Details)
$ in Millions
3 Months Ended
Aug. 31, 2018
USD ($)
Service Revenue [Abstract]  
Balance, beginning of period $ 46.4
Deferral of revenue 6.1
Recognition of unearned revenue (6.8)
Balance, end of period $ 45.7