Investment Securities - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of available-for-sale securities	$ 872,774,000		$ 872,774,000		$ 563,209,000
Securities	852,213,000		852,213,000		557,176,000
Proceeds from sales of securities available-for-sale	9,700,000		9,752,000	$ 57,350,000
Gross gains on sale of available-for-sale derivatives	116,000		116,000	500,000
Gross losses on sale of available-for-sale derivatives	0		61,000	89,000
Fair value of securities sold under agreements to repurchase	187,600,000		187,600,000		302,900,000
Other-than-temporary impairment charge on available-for-sale securities			0	0
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	254,131,000		254,131,000		132,939,000
Securities held-to-maturity, fair value	245,477,000		245,477,000		131,822,000
Proceeds from sale of held-to-maturity securities	0	$ 0	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	754,862,000		754,862,000		479,927,000
Securities	735,199,000		735,199,000		473,491,000
Fair value of securities sold under agreements to repurchase	124,600,000		124,600,000		0
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	240,729,000		240,729,000		129,532,000
Securities held-to-maturity, fair value	$ 232,318,000		$ 232,318,000		$ 128,422,000