Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended
		Jul. 31, 2020	Jul. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance		$ (379.0)
Accumulated Other Comprehensive Income (Loss), Ending Balance		(360.9)
Foreign Currency Translation Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance		(50.5)	$ (35.5)
Reclassification adjustments		0.0	0.0
Current period credit (charge)		12.9	4.5
Income tax benefit (expense)		0.0	0.0
Accumulated Other Comprehensive Income (Loss), Ending Balance		(37.6)	(31.0)
Net Gains (Losses) on Cash Flow Hedging Derivatives [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	[1]	(185.6)	(40.4)
Reclassification adjustments	[1]	3.4	0.1
Current period credit (charge)	[1]	0.0	(53.0)
Income tax benefit (expense)	[1]	(0.8)	12.1
Accumulated Other Comprehensive Income (Loss), Ending Balance	[1]	(183.0)	(81.2)
Pension and Other Postretirement Liabilities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	[2]	(146.7)	(110.0)
Reclassification adjustments	[2]	2.2	1.4
Current period credit (charge)	[2]	0.0	0.0
Income tax benefit (expense)	[2]	(0.5)	(0.3)
Accumulated Other Comprehensive Income (Loss), Ending Balance	[2]	(145.0)	(108.9)
Unrealized Gain (Loss) on Available-for-Sale Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance		3.8	4.1
Reclassification adjustments		0.0	0.0
Current period credit (charge)		1.1	0.4
Income tax benefit (expense)		(0.2)	(0.1)
Accumulated Other Comprehensive Income (Loss), Ending Balance		4.7	4.4
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance		(379.0)	(181.8)
Reclassification adjustments		5.6	1.5
Current period credit (charge)		14.0	(48.1)
Income tax benefit (expense)		(1.5)	11.7
Accumulated Other Comprehensive Income (Loss), Ending Balance		$ (360.9)	$ (216.7)

[1] The reclassification from accumulated other comprehensive income (loss) to interest expense was related to terminated interest rate contracts. The current period charge in 2020 relates to losses on the interest rate contracts entered into in November 2018 and June 2018 that were terminated in 2020. For additional information, see Note 8: Derivative Financial Instruments.

[2] Amortization of net losses and prior service costs was reclassified from accumulated other comprehensive income (loss) to other income (expense) – net.