Securitization - Schedule of Consolidated Statements of Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Variable Interest Entity [Line Items]
Operating revenues	$ 1,950,502	$ 1,647,227	$ 3,126,501	$ 2,805,694
Operation and maintenance expense	(233,296)	(199,899)	(440,340)	(366,244)
Amortization expense	(182,750)	(165,087)	(363,283)	(329,695)
Interest expense, net	(50,014)	(55,442)	(102,939)	(107,317)
Income before income taxes	603,097	512,235	1,034,286	877,351
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Operating revenues	2,951	3,469	6,344	6,802
Operation and maintenance expense	(216)	(224)	(277)	(224)
Amortization expense	(1,689)	(2,103)	(3,953)	(4,269)
Interest expense, net	(1,046)	(1,142)	(2,114)	(2,309)
Income before income taxes	$ 0	$ 0	$ 0	$ 0