Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 157,916	$ 144,786	$ 121,253
Securities available for sale:
Net unrealized holding (loss) gain arising during the period	(32,816)	(17,848)	5,347
Reclassification adjustment for net gain realized in net income	(6)	(75)	(160)
Income tax benefit (expense)	9,244	6,811	(1,971)
Net change in unrealized (loss) gain on securities available for sale	(23,578)	(11,112)	3,216
Defined benefit pension plan obligation:
Net unrealized holding (loss) gain arising during the period	(329)	329	0
Income tax benefit (expense)	125	(125)	0
Net change in defined benefit pension plan obligation	(204)	204	0
Other comprehensive (loss) income, net of tax	(23,782)	(10,908)	3,216
Comprehensive income	$ 134,134	$ 133,878	$ 124,469