CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 51,394	$ 44,112	$ 152,335	$ 127,428
Other comprehensive income (loss) net of tax:
Net unrealized gain (loss) on available-for-sale investment securities	(4,851)	3,171	(18,282)	(8,224)
Reclassification adjustment of net gain (loss) from sale of available-for-sale securities included in net income	0	0	0	968
Related tax benefit (expense)	1,104	(1,165)	5,506	2,667
Other comprehensive income (loss) on available-for-sale securities, net of tax	(3,747)	2,006	(12,776)	(4,589)
Net unrealized gain (loss) on cash flow hedges of borrowings	3,865	(2,856)	20,887	28,810
Related tax benefit (expense)	(879)	1,049	(4,989)	(10,587)
Other comprehensive income (loss) on cash flow hedges of borrowings, net of tax	2,986	(1,807)	15,898	18,223
Other comprehensive income (loss) net of tax	(761)	199	1,350	13,634
Other comprehensive income (loss), including cumulative effect adjustment, net of tax			3,122
Comprehensive income	$ 50,633	$ 44,311	$ 155,457	$ 141,062