Leases - Lease term, discount rate and supplemental cash flow information (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Leases
Weighted-average remaining lease term (years)	18 years 8 months 12 days		22 years 7 months 6 days
Weighted-average discount rate	2.27%		2.75%
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 710	$ 524
ROU assets obtained in exchange for lease obligations:
Operating leases	$ 2,037	$ 7,506