Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 144,786	$ 121,253	$ 109,065
Securities available for sale:
Net unrealized holding (loss) gain arising during the year	(17,848)	5,347	13,979
Reclassification adjustment for net (gain) realized in net income	(75)	(160)	(310)
Income tax benefit (expense)	6,811	(1,971)	(5,194)
Net change in unrealized (losses) gains on securities available for sale	(11,112)	3,216	8,475
Defined benefit pension plan obligation:
Net unrealized holding gain arising during the year	329	0	0
Income tax (expense)	(125)	0	0
Net change in defined benefit pension plan obligation	204	0	0
Other comprehensive (loss) income, net of tax	(10,908)	3,216	8,475
Comprehensive income	$ 133,878	$ 124,469	$ 117,540