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Pensions and Other Postretirement Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Defined Benefit Pension Plans [Member]      
Components of net periodic benefit cost:      
Service cost $ 5.2 $ 12.7 $ 17.8
Interest cost 21.6 25.3 27.7
Expected return on plan assets (28.8) (29.3) (32.9)
Amortization of prior service cost (credit) 0.9 1.1 0.7
Amortization of net actuarial loss (gain) 11.5 13.8 10.9
Curtailment loss (gain) 0.0 0.0 (6.5)
Settlement loss (gain) 2.3 (0.7) 0.0
Net periodic benefit cost 12.7 22.9 17.7
Other Postretirement Benefits [Member]      
Components of net periodic benefit cost:      
Service cost 2.0 2.3 2.3
Interest cost 2.1 2.6 2.8
Expected return on plan assets 0.0 0.0 0.0
Amortization of prior service cost (credit) (1.4) (1.5) (1.1)
Amortization of net actuarial loss (gain) (0.3) (0.2) (0.3)
Curtailment loss (gain) 0.0 0.0 (0.3)
Settlement loss (gain) 0.0 0.0 0.0
Net periodic benefit cost $ 2.4 $ 3.2 $ 3.4