Securities Available for Sale - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	$ 719,607,000	$ 1,051,128,000
Securities available for sale, at fair value	710,570,000	1,034,758,000
Proceeds from available-for-sale securities	92,000	11,500,000	$ 187,400,000	$ 164,200,000
Available-for-sale securities, gross realized gains	0	116,000	1,500,000	1,100,000
Proceeds from sale and maturity of debt securities, available for sale	0	2,400,000	17,200,000	5,400,000
Available-for-sale securities, gross realized losses	(60,000)		(3,200,000)	(743,000)
Securities available-for-sale sold under agreements	$ 301,100,000	$ 232,700,000
Number of unrealized loss positions	111	151
Other-than-temporary impairment loss, debt securities, available for sale		$ 0	$ 0	$ 0
Mortgage-backed securities and collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, amortized cost	$ 615,924,000	934,631,000
Securities available for sale, at fair value	606,612,000	920,007,000
Securities available-for-sale sold under agreements	$ 141,000,000	$ 187,000,000