Variable Interest Entity - Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Variable Interest Entity [Line Items]
Operating revenues	$ 4,165,187	$ 4,275,357	$ 4,201,662
Operation and maintenance expense	(819,137)	(764,906)	(710,161)
Amortization expense	(669,972)	(604,327)	(535,655)
Interest expense, net	(190,632)	(137,281)	(102,811)
Income before income taxes	1,235,776	999,641	$ 851,908
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Operating revenues	13,660	2,743
Operation and maintenance expense	(427)	0
Amortization expense	(8,715)	(1,398)
Interest expense, net	(4,518)	(1,345)
Income before income taxes	$ 0	$ 0