Debt - Narrative (Details)							6 Months Ended
	Mar. 31, 2022 USD ($) credit_facility	Jan. 14, 2022 USD ($)	Oct. 01, 2021 USD ($)	Apr. 01, 2021 USD ($)	Mar. 31, 2021	Mar. 30, 2021 USD ($)	Mar. 31, 2022 USD ($) credit_facility	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)
Line Of Credit Facility [Line Items]
Proceeds from issuance of long-term debt							$ 798,802,000	$ 2,797,346,000
Maximum debt-to-total-capitalization ratio							70.00%
Debt-to-total-capitalization ratio	0.48						0.48
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
Unsecured 1.50% Senior Notes, due 2031
Line Of Credit Facility [Line Items]
Debt instrument, face amount			$ 600,000,000
Interest rate	1.50%		2.85%				1.50%
Effective Interest Rate			2.58%
Proceeds from issuance of long-term debt			$ 589,800,000
Unsecured 2.625% Senior Notes, due 2029
Line Of Credit Facility [Line Items]
Debt instrument, face amount		$ 200,000,000
Interest rate	2.625%	2.625%					2.625%
Effective Interest Rate		2.54%
Floating-rate term loan, due April 2022
Line Of Credit Facility [Line Items]
Repayments of debt		$ 200,800,000
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
Accordion feature	250,000,000						250,000,000
Maximum borrowing capacity post accordion feature	1,750,000,000						1,750,000,000
$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						1,000,000,000
Outstanding borrowings	0						0
$50 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity				$ 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 | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread					0.00%
Base Rate | Five Year Unsecured Revolving Credit Agreement | 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%
SOFR | Five Year Unsecured Revolving Credit Agreement | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread					0.75%
SOFR | Five Year Unsecured Revolving Credit Agreement | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread					1.25%
SOFR | $900 Million Revolving Credit Facility | Revolving Credit Facility | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread					0.75%
SOFR | $900 Million Revolving Credit Facility | Revolving Credit Facility | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread					1.25%