Debt - Narrative (Details)				6 Months Ended
	Mar. 31, 2021 USD ($) credit_facility	Mar. 09, 2021 USD ($)	Oct. 01, 2020 USD ($)	Mar. 31, 2021 USD ($) credit_facility	Mar. 30, 2021 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Line Of Credit Facility [Line Items]
Proceeds from issuance of long-term debt				$ 2,797,346,000		$ 799,450,000
Maximum debt-to-total-capitalization ratio				70.00%
Debt-to-total-capitalization ratio	0.49			0.49
Minimum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio				50.00%
Outstanding indebtedness	$ 15,000,000			$ 15,000,000
Maximum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio				60.00%
Outstanding indebtedness	100,000,000			$ 100,000,000
Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	$ 2,500,000,000			$ 2,500,000,000
Number of credit facilities | credit_facility	4			4
Senior Notes Due 2023
Line Of Credit Facility [Line Items]
Debt instrument, face amount		$ 2,200,000,000
Proceeds net of issuance cost		$ 2,200,000,000
Redemption price (in percent)		100.00%
Unsecured 0.625% Senior Notes, due 2023
Line Of Credit Facility [Line Items]
Debt instrument, face amount		$ 1,100,000,000
Interest rate	0.625%	0.625%		0.625%
Effective Interest Rate		0.834%
Floating-rate Senior Notes, due 2023
Line Of Credit Facility [Line Items]
Debt instrument, face amount		$ 1,100,000,000
Unsecured 1.50% Senior Notes, due 2031
Line Of Credit Facility [Line Items]
Debt instrument, face amount			$ 600,000,000
Interest rate	1.50%		1.50%	1.50%
Effective Interest Rate			1.71%
Proceeds from issuance of long-term debt			$ 592,300,000
Five Year Unsecured Revolving Credit Agreement
Line Of Credit Facility [Line Items]
Outstanding commercial paper	$ 0			$ 0			$ 0
Five Year Unsecured Revolving Credit Agreement | Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,500,000,000			1,500,000,000	$ 1,500,000,000
Debt agreement term	5 years				5 years
Accordion feature	$ 250,000,000			250,000,000
Maximum borrowing capacity post accordion feature	1,750,000,000			1,750,000,000
$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
$900 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	$ 900,000,000			900,000,000
Debt agreement term	3 years
Accordion feature	$ 100,000,000			100,000,000
Maximum borrowing capacity post accordion feature	1,000,000,000.0			1,000,000,000.0
Outstanding borrowings	0			0
$50 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	50,000,000			$ 50,000,000
Debt agreement term				364 days
Outstanding borrowings	0			$ 0
$50 Million Revolving Credit Facility
Line Of Credit Facility [Line Items]
Outstanding borrowings	0			0
$50 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	50,000,000			$ 50,000,000
Debt agreement term				364 days
Remaining borrowing capacity	$ 44,400,000			$ 44,400,000
Base Rate | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread	0.00%
Base Rate | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread	0.25%
Base Rate | $900 Million Revolving Credit Facility | Revolving Credit Facility | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread	0.00%
Base Rate | $900 Million Revolving Credit Facility | Revolving Credit Facility | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread	0.25%
LIBOR | Floating-rate Senior Notes, due 2023
Line Of Credit Facility [Line Items]
Interest rate spread		0.38%
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread	0.75%
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread	1.25%
LIBOR | $900 Million Revolving Credit Facility | Revolving Credit Facility | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread	0.75%
LIBOR | $900 Million Revolving Credit Facility | Revolving Credit Facility | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread	1.25%