Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 257,426	$ 236,330	$ 183,615
Other comprehensive income (loss), net of tax:
Net unrealized gain (loss) during the period on available-for-sale debt securities, net of tax of $2,493, $36,908 and $(2,020)	(9,360)	(123,077)	6,762
Reclassification adjustment of net (gain) loss included in net income during the period from sale of available-for-sale securities, net of tax of $(9), $(23) and $(3)	26	76	11
Net unrealized gain (loss) from investment securities, net of reclassification adjustment	(9,334)	(123,001)	6,773
Net unrealized gain (loss) during the period on borrowing cash flow hedges, net of tax of $(654), $(31,805) and $(17,003)	3,774	105,697	56,924
Reclassification adjustment of net (gain) loss included in net income during the period from hedging derivatives, net of tax of $0, $0 and $3,245	0	0	(10,865)
Net unrealized gain (loss) in cash flow hedging instruments, net of reclassification adjustment	3,774	105,697	46,059
Other comprehensive income (loss)	(5,560)	(17,304)	52,832
Comprehensive income	$ 251,866	$ 219,026	$ 236,447