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Long-term and Short-term Debt - Summary of Long Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Unamortized discount and other	$ (62.2)		$ (50.8)
Total	2,239.9		1,225.2
Less current portion	(298.8)		0.0
Long-term debt	1,941.1		1,225.2
Notes \| 2.050% notes, payable in March 2020
Debt Instrument [Line Items]
Long-term debt, gross	$ 298.8		294.6
Stated interest rate	2.05%
Notes \| 2.875% notes, payable in March 2025
Debt Instrument [Line Items]
Long-term debt, gross	$ 303.3		281.4
Stated interest rate	2.875%
Notes \| 6.70% debentures, payable in January 2028
Debt Instrument [Line Items]
Long-term debt, gross	$ 250.0		250.0
Stated interest rate	6.70%
Notes \| 3.500% notes, payable in March 2029
Debt Instrument [Line Items]
Long-term debt, gross	$ 425.0		0.0
Stated interest rate	3.50%	3.50%
Notes \| 6.25% debentures, payable in December 2037
Debt Instrument [Line Items]
Long-term debt, gross	$ 250.0		250.0
Stated interest rate	6.25%
Notes \| 4.200% notes, payable in March 2049
Debt Instrument [Line Items]
Long-term debt, gross	$ 575.0		0.0
Stated interest rate	4.20%	4.20%
Notes \| 5.20% debentures, payable in January 2098
Debt Instrument [Line Items]
Long-term debt, gross	$ 200.0		$ 200.0
Stated interest rate	5.20%