Long-term and Short-term Debt - Long-term and Short-term Debt (Details)		1 Months Ended	2 Months Ended	3 Months Ended		12 Months Ended
	Mar. 01, 2019 USD ($)	Nov. 12, 2018 USD ($)	Dec. 31, 2018	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2019 USD ($)	Nov. 13, 2018 USD ($)	Feb. 28, 2015 USD ($)
Long-term debt
Unrealized gain (loss) on investments				$ 7,000,000.0
Revolving credit facility
Interest payments						$ 97,500,000	$ 75,500,000	$ 74,200,000
Short-term Debt
Commercial paper borrowings outstanding				$ 0		$ 0	550,000,000.0
3.500% notes, payable in March 2029
Long-term debt
Stated interest rate				3.50%		3.50%
4.200% notes, payable in March 2049
Long-term debt
Stated interest rate				4.20%		4.20%
Aggregate principal amount of notes issued
Long-term debt
Long-term debt issued											$ 600,000,000.0
2.050% notes, payable in March 2020
Long-term debt
Stated interest rate				2.05%		2.05%
Effective floating interest rate											2.576%
2.875% notes, payable in March 2025
Long-term debt
Stated interest rate				2.875%		2.875%
Effective floating interest rate											2.986%
Notes Payable, Other Payables
Long-term debt
Long-term debt issued									$ 1,000,000,000
Notes Payable, Other Payables | 3.500% notes, payable in March 2029
Long-term debt
Long-term debt issued									$ 425,000,000.0
Stated interest rate									3.50%
Notes Payable, Other Payables | 4.200% notes, payable in March 2049
Long-term debt
Long-term debt issued									$ 575,000,000.0
Stated interest rate									4.20%
Proceeds from issuance of debt					$ 987,600,000
Line of Credit | Revolving Credit Facility
Revolving credit facility
Line of credit facility, current borrowing capacity		$ 1,000,000,000.0								$ 1,250,000,000
Debt instrument, term		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
Line of Credit | Revolving Credit Facility | Credit facility, November 2023
Revolving credit facility
Debt instrument, term			5 years
Treasury locks
Long-term debt
Derivative, gain (loss) on derivative, net	$ 35,700,000
Commercial Paper
Short-term Debt
Weighted average interest rate of commercial paper outstanding							2.27%