Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Numerator: Basic and Diluted
Net income (loss)	$ 41,975	$ 45,087	$ (8,683)
Dividends paid ($0.88 per share for the year ended September 30, 2021, 2022 and 2023, respectively)	(11,190)	(11,062)	(11,245)
Undistributed income (loss) - Basic	30,785	34,025	(19,928)
Undistributed income (loss) - Diluted	$ 30,785	$ 34,025	$ (19,928)
Percentage allocated to common shares - Basic	99.20%	99.00%	100.00%
Percentage allocated to common shares - Diluted	99.20%	99.00%	100.00%
Undistributed income (loss) allocated to common shares - Basic	$ 30,544	$ 33,699	$ (19,928)
Undistributed income (loss) allocated to common shares - Diluted	30,544	33,699	(19,928)
Dividends paid on common shares outstanding	11,103	10,955	11,098
Net income (loss) available to common shares - Basic	41,647	44,654	(8,830)
Net income (loss) available to common shares - Diluted	$ 41,647	$ 44,654	$ (8,830)
Denominator: Basic and Diluted
Weighted average shares outstanding - Basic (in shares)	12,566,843	12,346,025	12,499,609
Adjustment for dilutive potential common shares (in shares)	213,435	159,475
Weighted average shares outstanding - Diluted (in shares)	12,780,278	12,505,500	12,499,609
Basic net income (loss) per share (in dollars per share)	$ 3.31	$ 3.62	$ (0.71)
Diluted net income (loss) per share (in dollars per share)	$ 3.26	$ 3.57	$ (0.71)