Debt - Narrative (Details)					6 Months Ended
	Apr. 30, 2020 USD ($)	Apr. 23, 2020 USD ($) credit_facility	Apr. 09, 2020 USD ($)	Oct. 02, 2019 USD ($)	Mar. 31, 2020 USD ($) credit_facility	Apr. 01, 2020 USD ($)	Sep. 30, 2019 USD ($)
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					$ 1,500,000,000
Short-term debt					$ 199,923,000		$ 464,915,000
Maximum debt-to-total-capitalization ratio					70.00%
Debt-to-total-capitalization ratio					0.44
Minimum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio					50.00%
Outstanding indebtedness					$ 15,000,000
Maximum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio					60.00%
Outstanding indebtedness					$ 100,000,000
Revolving Credit Facility
Line Of Credit Facility [Line Items]
Number of credit facilities | credit_facility					3
Unsecured 2.625% Senior Notes, due 2029
Line Of Credit Facility [Line Items]
Debt instrument, face amount				$ 300,000,000
Interest rate				2.625%	2.625%
Effective interest rate					2.72%
Unsecured 3.375% Senior Notes, due 2049
Line Of Credit Facility [Line Items]
Debt instrument, face amount				$ 500,000,000
Interest rate				3.375%	3.375%
Debt issuance cost				$ 791,700,000
Effective interest rate					3.42%
Five Year Unsecured Revolving Credit Agreement
Line Of Credit Facility [Line Items]
Outstanding commercial paper					$ 199,900,000		$ 464,900,000
Five Year Unsecured Revolving Credit Agreement | Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					$ 1,500,000,000
Debt agreement term					5 years
Accordion feature					$ 250,000,000
Maximum borrowing capacity post accordion feature					1,750,000,000
$25 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					$ 25,000,000
Debt agreement term					364 days
Outstanding borrowings					$ 0
$10 Million Revolving Credit Note | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity					$ 10,000,000
Debt agreement term					364 days
Remaining borrowing capacity					$ 4,400,000
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread					0.00%
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread					1.25%
Subsequent Event | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	$ 2,200,000,000
Number of credit facilities | credit_facility		4
Subsequent Event | $200 Million Term Loan Agreement
Line Of Credit Facility [Line Items]
Debt agreement term			2 years
Subsequent Event | Five Year Unsecured Revolving Credit Agreement | Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	1,500,000,000
Subsequent Event | $50 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity						$ 50,000,000
Subsequent Event | $50 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	$ 50,000,000
Debt agreement term	364 days
Subsequent Event | $600 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		$ 600,000,000
Debt agreement term		364 days
Subsequent Event | LIBOR | $600 Million Revolving Credit Facility | Revolving Credit Facility | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread		0.00%
Subsequent Event | LIBOR | $600 Million Revolving Credit Facility | Revolving Credit Facility | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread		1.25%
Secured Debt | Subsequent Event | $200 Million Term Loan Agreement
Line Of Credit Facility [Line Items]
Debt instrument, face amount			$ 200,000,000
Secured Debt | Subsequent Event | LIBOR | $200 Million Term Loan Agreement
Line Of Credit Facility [Line Items]
Interest rate spread			1.25%