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Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($)
$ in Millions
12 Months Ended
May 31, 2019
May 31, 2018
May 31, 2017
Revenue Recognition      
Favorable cumulative catch-up adjustments $ 8.0    
Unfavorable cumulative catch-up adjustments $ 2.1    
Net cumulative catch-up adjustments   $ 3.6 $ 8.5
Incremental costs of obtaining a contract true    
Remaining performance obligations true