Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Number of unrealized loss positions	84	88
Amortized cost	$ 239,618,000	$ 262,143,000
Securities held-to-maturity, fair value	236,125,000	254,841,000
Proceeds from sale of held-to-maturity securities		0	$ 0	$ 0
Proceeds from maturities, prepayments and calls of held-to-maturity securities	0	5,400,000	769,000	0
Securities available-for-sale sold under agreements	301,100,000	232,700,000
Transfer of securities from available for sale securities	0	0	103,680,000	0
Transfer of securities from available for sale securities, fair value			103,300,000
Other-than-temporary impairment loss, debt securities, held-to-maturity, before tax		0	$ 0	$ 0
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	231,216,000	238,739,000
Securities held-to-maturity, fair value	227,781,000	231,600,000
Securities available-for-sale sold under agreements	$ 141,000,000	$ 187,000,000