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Long-term and Short-term Debt - Narrative (Details) - USD ($)		1 Months Ended	6 Months Ended
	Mar. 01, 2019	Mar. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Issuance of long-term debt, net of discount and issuance costs			$ 987,600,000	$ 0
Commercial paper borrowings outstanding		$ 0	0		$ 550,000,000.0
Weighted average interest rate of commercial paper outstanding					2.27%
Notes
Debt Instrument [Line Items]
Long-term debt		1,000,000,000.0	1,000,000,000.0
Issuance of long-term debt, net of discount and issuance costs		987,600,000
Notes \| 3.500% notes, payable in March 2029
Debt Instrument [Line Items]
Long-term debt		$ 425,000,000.0	$ 425,000,000.0
Stated interest rate		3.50%	3.50%
Notes \| 4.200% notes, payable in March 2049
Debt Instrument [Line Items]
Long-term debt		$ 575,000,000.0	$ 575,000,000.0
Stated interest rate		4.20%	4.20%
Treasury locks related to 2019 debt issuance
Debt Instrument [Line Items]
Loss on settlement of treasury lock	$ 35,700,000