CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 34,852	$ 53,854	$ 139,095	$ 157,894
Other comprehensive income (loss) net of tax:
Net unrealized gain (loss) on available-for-sale investment securities	15,966	14,577	28,902	31,677
Reclassification adjustment of net (gain) loss from sale of available-for-sale securities included in net income	0	0	(15,028)	(9)
Related tax benefit (expense)	(3,672)	(3,316)	(3,260)	(7,204)
Other comprehensive income (loss) on available-for-sale securities, net of tax	12,294	11,261	10,614	24,464
Net unrealized gain (loss) on borrowings cash flow hedges	(8,045)	(10,043)	(17,358)	(26,692)
Related tax benefit (expense)	1,850	2,285	4,012	6,071
Other comprehensive income (loss) on cash flow hedges of borrowings, net of tax	(6,195)	(7,758)	(13,346)	(20,621)
Other comprehensive income (loss) net of tax	6,099	3,503	(2,732)	3,843
Comprehensive income	$ 40,951	$ 57,357	$ 136,363	$ 161,737