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Debt - Narrative (Details) |  |  |  |  |  |  |  |  | 1 Months Ended | 12 Months Ended |
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Jun. 26, 2025 USD ($) | Apr. 01, 2025 USD ($) | Mar. 31, 2025 USD ($) | Oct. 01, 2024 USD ($) | Jun. 21, 2024 USD ($) | Mar. 28, 2024 USD ($) | Mar. 27, 2024 USD ($) | Oct. 10, 2023 USD ($) | Sep. 30, 2025 USD ($) credit_facility | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2025 USD ($) credit_facility | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Oct. 01, 2025 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance long term debt |  |  |  |  |  |  |  |  |  |  |  | $ 1,143,447,000 | $ 1,240,204,000 | $ 797,258,000 |
Offering received |  |  |  |  |  |  |  |  |  |  |  | $ 122,874,000 | 231,138,000 | $ 171,145,000 |
Ratio of total debt to total capitalization maximum |  |  |  |  |  |  |  |  |  |  |  | 70.00% |  |  |
Ratio of indebtedness to net capital |  |  |  |  |  |  |  |  | 0.41 |  |  | 0.41 |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding indebtedness |  |  |  |  |  |  |  |  | $ 15,000,000 |  |  | $ 15,000,000 |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding indebtedness |  |  |  |  |  |  |  |  | 100,000,000 |  |  | $ 100,000,000 |  |  |
Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Offering received |  |  |  |  |  |  |  |  | 122,900,000 | $ 231,100,000 | $ 171,100,000 |  |  |  |
Unsecured 5.45% Senior Notes, Due January 2056 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance long term debt |  |  |  |  |  |  |  |  | $ 589,800,000 |  |  |  |  |  |
Unsecured 5.45% Senior Notes, Due January 2056 | Senior Notes | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 600,000,000
Interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.45%
Effective rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.85%
Unsecured 5.20% Senior Notes, due August 2035 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  | 5.20% |  |  | 5.20% |  |  |
Unsecured 5.20% Senior Notes, due August 2035 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount | $ 500,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate | 5.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effective rate | 5.35% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance long term debt | $ 493,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured 5.00% Senior Notes, due December 2054 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  | 5.00% |  |  | 5.00% |  |  |
Unsecured 5.00% Senior Notes, due December 2054 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount |  |  |  | $ 650,000,000 |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  | 5.00% |  |  |  |  |  |  |  |  |  |  |
Effective rate |  |  |  | 3.90% |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance long term debt |  |  |  | $ 639,400,000 |  |  |  |  |  |  |  |  |  |  |
Unsecured 5.90% Senior Notes, due November 2033 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  | 5.90% |  |  | 5.90% |  |  |
Unsecured 5.90% Senior Notes, due November 2033 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount |  |  |  |  | $ 325,000,000 |  |  | $ 400,000,000 |  |  |  |  |  |  |
Interest rate |  |  |  |  | 5.90% |  |  | 5.90% |  |  |  |  |  |  |
Effective rate |  |  |  |  | 5.17% |  |  | 4.35% |  |  |  |  |  |  |
Proceeds from issuance long term debt |  |  |  |  | $ 339,000,000 |  |  |  |  |  |  |  |  |  |
Unsecured 6.20% Senior Notes, due November 2053 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  | 6.20% |  |  | 6.20% |  |  |
Unsecured 6.20% Senior Notes, due November 2053 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount |  |  |  |  |  |  |  | $ 500,000,000 |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  | 6.20% |  |  |  |  |  |  |
Effective rate |  |  |  |  |  |  |  | 5.56% |  |  |  |  |  |  |
Unsecured Senior Notes Due 2033 and Unsecured Senior Notes Due 2053 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of debt |  |  |  |  |  |  |  | $ 889,400,000 |  |  |  |  |  |  |
Five Year Unsecured Revolving Credit Agreement, Maturing On March 28, 2029 | Minimum | Base Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 0.00% |  |  |
Five Year Unsecured Revolving Credit Agreement, Maturing On March 28, 2029 | Minimum | Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 0.75% |  |  |
Five Year Unsecured Revolving Credit Agreement, Maturing On March 28, 2029 | Maximum | Base Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 0.25% |  |  |
Five Year Unsecured Revolving Credit Agreement, Maturing On March 28, 2029 | Maximum | Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 1.25% |  |  |
Five Year Unsecured Revolving Credit Agreement, Maturing On March 28, 2029 | Commercial Paper |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  |  |  | $ 1,500,000,000 |  |  | $ 1,500,000,000 |  |  |
Credit facility, accordion feature |  |  |  |  |  |  |  |  | 250,000,000 |  |  | 250,000,000 |  |  |
Loan increase potential |  |  |  |  |  |  |  |  | 1,750,000,000 |  |  | 1,750,000,000 |  |  |
Five Year Unsecured Revolving Credit Agreement, Maturing On March 28, 2029 | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  |  |  | $ 3,100,000,000 |  |  | $ 3,100,000,000 |  |  |
Number of credit facilities | credit_facility |  |  |  |  |  |  |  |  | 4 |  |  | 4 |  |  |
Five Year Unsecured Revolving Credit Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  |  |  |  |  |  |  |  | $ 0 |  |  | $ 0 |  |  |
Five Year Unsecured Revolving Credit Agreement | Commercial Paper |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  | $ 1,500,000,000 |  |  |  |  |  |  |  |
Debt term |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |
$1.5 Billion Revolving Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  | $ 1,500,000,000 |  |  |  |  |  |  |  |  |
Debt term |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |
Credit facility, accordion feature |  |  |  |  |  | $ 250,000,000 |  |  |  |  |  |  |  |  |
Loan increase potential |  |  |  |  |  | $ 1,750,000,000 |  |  |  |  |  |  |  |  |
Line of credit borrowings outstanding |  |  |  |  |  |  |  |  | 0 | 0 |  | $ 0 | 0 |  |
$1.5 Billion Revolving Credit Facility | Revolving Credit Facility | Minimum | Base Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 0.00% |  |  |
$1.5 Billion Revolving Credit Facility | Revolving Credit Facility | Minimum | Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 0.75% |  |  |
$1.5 Billion Revolving Credit Facility | Revolving Credit Facility | Maximum | Base Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 0.25% |  |  |
$1.5 Billion Revolving Credit Facility | Revolving Credit Facility | Maximum | Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread percent |  |  |  |  |  |  |  |  |  |  |  | 1.25% |  |  |
$900 Million Revolving Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit borrowings outstanding |  |  |  |  |  |  |  |  | 0 | 0 |  | $ 0 | 0 |  |
$50 Million Bank Loan Agreement | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  | $ 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt term |  | 364 days |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit borrowings outstanding |  |  |  |  |  |  |  |  | 0 | $ 0 |  | 0 | $ 0 |  |
$50 Million Revolving Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  | $ 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Debt term |  |  | 364 days |  |  |  |  |  |  |  |  |  |  |  |
Line of credit borrowings outstanding |  |  |  |  |  |  |  |  | 0 |  |  | 0 |  |  |
Remaining borrowing capacity |  |  |  |  |  |  |  |  | $ 44,400,000 |  |  | $ 44,400,000 |  |  |

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