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Service Revenue (Summary of Changes in Deferred Revenue Related to Material Right Performance Obligations) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Nov. 30, 2018
Nov. 30, 2018
Service Revenue [Abstract]    
Balance, beginning of period $ 45.7 $ 46.4
Deferral of revenue 7.1 13.2
Recognition of unearned revenue (6.8) (13.6)
Balance, end of period $ 46.0 $ 46.0