Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		6 Months Ended
		Oct. 31, 2019	Oct. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance		$ (181.8)
Accumulated Other Comprehensive Income (Loss), Ending Balance		(248.4)
Foreign Currency Translation Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance		(35.5)	$ (16.4)
Reclassification adjustments		0.0	0.0
Current period credit (charge)		5.1	(11.7)
Income tax benefit (expense)		0.0	0.0
Accumulated Other Comprehensive Income (Loss), Ending Balance		(30.4)	(28.1)
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]	(40.4)	(2.9)
Reclassification adjustments	[1]	0.2	0.2
Current period credit (charge)	[1]	(97.1)	10.5
Income tax benefit (expense)	[1]	22.3	(2.5)
Accumulated Other Comprehensive Income (Loss), Ending Balance	[1]	(115.0)	5.3
Pension and Other Postretirement Liabilities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance	[2]	(110.0)	(101.0)
Reclassification adjustments	[2]	3.2	4.3
Current period credit (charge)	[2]	0.0	0.0
Income tax benefit (expense)	[2]	(0.7)	(1.1)
Accumulated Other Comprehensive Income (Loss), Ending Balance	[2]	(107.5)	(97.8)
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		4.1	3.6
Reclassification adjustments		0.0	0.0
Current period credit (charge)		0.5	0.8
Income tax benefit (expense)		(0.1)	(0.2)
Accumulated Other Comprehensive Income (Loss), Ending Balance		4.5	4.2
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Beginning Balance		(181.8)	(116.7)
Reclassification adjustments		3.4	4.5
Current period credit (charge)		(91.5)	(0.4)
Income tax benefit (expense)		21.5	(3.8)
Accumulated Other Comprehensive Income (Loss), Ending Balance		$ (248.4)	$ (116.4)

[1] The reclassification from accumulated other comprehensive income (loss) to interest expense was related to terminated interest rate contracts. The current period credit (charge) relates to the unrealized gains (losses) on the interest rate contracts entered into in November 2018 and June 2018. For additional information, see Note 10: 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.