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Other Financial Instruments and Fair Value Measurements - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Fair Value Disclosures [Abstract]      
Company's Municipal bond mature in 2019 $ 1.4    
Company's Municipal bond mature in 2020 1.7    
Company's Municipal bond mature in 2021 4.9    
Company's Municipal bond mature in 2022 1.0    
Company's Municipal bond mature in 2023 and beyond 27.1    
Impairment charges 176.9    
Other intangible assets impairment charges 31.9 $ 133.2 $ 0.0
Goodwill impairment charge $ 145.0 [1] $ 0.0 $ 0.0
[1] There have been no goodwill impairment charges recognized in prior periods.