Securitization - Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Variable Interest Entity [Line Items]
Operating revenues	$ 4,702,755	$ 4,165,187	$ 4,275,357
Operation and maintenance expense	(902,942)	(819,137)	(764,906)
Amortization expense	(734,745)	(669,972)	(604,327)
Interest expense, net	(171,678)	(190,632)	(137,281)
Income before income taxes	1,478,034	1,235,776	999,641
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Operating revenues	12,408	13,660	2,743
Operation and maintenance expense	(559)	(427)	0
Amortization expense	(7,717)	(8,715)	(1,398)
Interest expense, net	(4,132)	(4,518)	(1,345)
Income before income taxes	$ 0	$ 0	$ 0