Investment Securities - Narrative (Details) - USD ($)	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of available-for-sale securities	$ 719,607,000		$ 563,209,000
Securities	710,570,000		557,176,000
Proceeds from sales of securities available-for-sale	92,000	$ 58,047,000
Gross gains on sale of available-for-sale derivatives	0	500,000
Gross losses on sale of available-for-sale derivatives	60,000	89,000
Fair value of securities sold under agreements to repurchase	282,600,000		302,900,000
Other-than-temporary impairment charge on available-for-sale securities	0	0
Schedule of Held-to-maturity Securities [Line Items]
Securities held-to-maturity, at amortized cost (fair value of $236,125 and $131,822 at December 31, 2017 and September 30, 2017, respectively)	239,618,000		132,939,000
Securities held-to-maturity, fair value	236,125,000		131,822,000
Proceeds from sale of held-to-maturity securities	0	0
Other-than-temporary impairment charge on held-to-maturity securities	0	$ 0
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of available-for-sale securities	615,924,000		479,927,000
Securities	606,612,000		473,491,000
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held-to-maturity, at amortized cost (fair value of $236,125 and $131,822 at December 31, 2017 and September 30, 2017, respectively)	231,216,000		129,532,000
Securities held-to-maturity, fair value	$ 227,781,000		$ 128,422,000