CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 37,950	$ 51,098	$ 103,653	$ 104,040
Other comprehensive income (loss) net of tax:
Net unrealized gain (loss) on available-for-sale investment securities	15,086	13,585	12,937	17,082
Reclassification adjustment of net (gain) loss from sale of available-for-sale securities included in net income	(15,028)	0	(15,028)	9
Related tax benefit (expense)	(13)	(3,091)	412	(3,889)
Other comprehensive income (loss) on available-for-sale securities, net of tax	45	10,494	(1,679)	13,202
Net unrealized gain (loss) on borrowings cash flow hedges	(12,428)	(6,150)	(9,313)	(16,650)
Related tax benefit (expense)	2,858	1,399	2,161	3,788
Other comprehensive income (loss) on cash flow hedges of borrowings, net of tax	(9,570)	(4,751)	(7,152)	(12,862)
Other comprehensive income (loss) net of tax	(9,525)	5,743	(8,831)	340
Comprehensive income	$ 28,425	$ 56,841	$ 94,822	$ 104,380