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Pensions and Other Postretirement Benefits (Details 8) - Significant Unobservable Inputs (Level 3) [Member] - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Change in plan assets:    
Fair value of plan assets at beginning of year $ 2.4 $ 3.2
Actual return on plan assets still held at reporting date 0.8 (0.8)
Fair value of plan assets at end of year $ 3.2 $ 2.4