Long-term and Short-term Debt - Long-term and Short-term Debt (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Feb. 28, 2015 USD ($)
Long-term debt
Fair value of interest rate swap contracts		$ 24,000,000
Revolving credit facility
Interest payments		75,500,000	$ 74,200,000	$ 69,200,000
Short-term Debt
Commercial paper borrowings outstanding		$ 550,000,000	$ 350,000,000
Weighted average interest rate of commercial paper outstanding		2.27%	1.26%
Aggregate Principal Amount of Notes Issues
Long-term debt
Long-term debt issued					$ 600,000,000
2.050% notes, payable in March 2020
Long-term debt
Effective floating interest rate					2.759%
2.875% notes, payable in March 2025
Long-term debt
Effective floating interest rate					3.169%
Line of Credit
Revolving credit facility
Borrowing capacity		$ 1,000,000,000	$ 1,000,000,000
Option to increase borrowing capacity of credit facility		350,000,000.0
Foreign Line of Credit
Revolving credit facility
Borrowing capacity		$ 156,000,000
Maximum | Line of Credit
Revolving credit facility
Maximum debt to total capital ratio required by debt covenants	0.60
Maximum ratio of EBITDA to interest expense under credit facility	3.0