Variable Interest Entity - Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Variable Interest Entity [Line Items]
Operating revenues	$ 4,275,357	$ 4,201,662	$ 3,407,490
Amortization expense	(604,327)	(535,655)	(477,977)
Interest charges	(137,281)	(102,811)	(83,554)
Income before income taxes	999,641	$ 851,908	$ 819,299
Variable Interest Entity, Primary Beneficiary | AEK
Variable Interest Entity [Line Items]
Operating revenues	2,743
Amortization expense	(1,398)
Interest charges	(1,345)
Income before income taxes	$ 0