Long-term and Short-term Debt - Long-term and Short-term Debt (Details)		1 Months Ended				2 Months Ended	12 Months Ended
	Mar. 01, 2019 USD ($)	Sep. 30, 2020 USD ($)	May 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Nov. 12, 2018 USD ($)	Dec. 31, 2018	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Apr. 30, 2020 USD ($)	Nov. 13, 2018 USD ($)	Feb. 28, 2015 USD ($)
Debt Instrument [Line Items]
Short-term debt		$ 24,600,000					$ 24,600,000	$ 0
Long-term debt
Proceeds from derivative instrument, financing activities			$ 22,000,000.0
Gain (loss) on hedging activity			$ 22,000,000.0
Revolving credit facility
Interest payments							$ 101,700,000	97,500,000	$ 75,500,000
Aggregate principal amount of notes issued
Long-term debt
Debt instrument, face amount												$ 600,000,000.0
4.200% notes, payable in March 2049
Long-term debt
Stated interest rate		4.20%					4.20%
3.500% notes, payable in March 2029
Long-term debt
Stated interest rate		3.50%					3.50%
Notes Payable, Other Payables
Long-term debt
Debt instrument, face amount				$ 1,000,000,000
Line of credit facility, current borrowing capacity										$ 1,250,000,000
Notes Payable, Other Payables | 4.200% notes, payable in March 2049
Long-term debt
Debt instrument, face amount				$ 575,000,000.0
Stated interest rate				4.20%
Proceeds from issuance of debt				$ 987,600,000
Notes Payable, Other Payables | 3.500% notes, payable in March 2029
Long-term debt
Debt instrument, face amount				$ 425,000,000.0
Stated interest rate				3.50%
Line of Credit | Revolving Credit Facility
Long-term debt
Line of credit facility, current borrowing capacity					$ 1,000,000,000.0						$ 1,250,000,000
Revolving credit facility
Debt instrument, term					5 years	5 years
Borrowing capacity											$ 750,000,000.0
Line of credit facility, fair value of amount outstanding		$ 0					$ 0	$ 0
Maximum ratio of EBITDA to interest expense under credit facility							3.0
Treasury locks
Long-term debt
Derivative, gain (loss) on derivative, net	$ 35,700,000
Interest Bearing Loan
Debt Instrument [Line Items]
Short-term debt		23,500,000					$ 23,500,000
Notes Payable to Banks
Long-term debt
Debt instrument, face amount										$ 400,000,000.0
Long-term debt, term										364 days
Repayments of debt		$ 400,000,000.0