Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Numerator: Basic and Diluted
Net income (loss)	$ 45,087	$ (8,683)	$ (6,478)
Dividends paid and accrued ($0.88 and $0.88 and $0.88 per share for the year ended September 30, 2020 and 2021 and 2022, respectively)	(11,062)	(11,245)	(11,158)
Undistributed income (loss) - Basic	34,025	(19,928)	(17,636)
Undistributed income (loss) - Diluted	$ 34,025	$ (19,928)	$ (17,636)
Percentage allocated to common shares	99.00%	100.00%	100.00%
Undistributed income (loss) allocated to common shares - Basic	$ 33,699	$ (19,928)	$ (17,636)
Undistributed income (loss) allocated to common shares - Diluted	33,699	(19,928)	(17,636)
Dividends paid on common shares outstanding	10,955	11,098	11,071
Net income (loss) available to common shares - Basic	44,654	(8,830)	(6,565)
Net income (loss) available to common shares - Diluted	$ 44,654	$ (8,830)	$ (6,565)
Denominator: Basic and Diluted
Weighted average shares outstanding - Basic (in shares)	12,346,025	12,499,609	12,470,664
Adjustment for dilutive potential common shares (in shares)	159,475
Weighted average shares outstanding - Diluted (in shares)	12,505,500	12,499,609	12,470,664
Basic net income (loss) per share (in dollars per share)	$ 3.62	$ (0.71)	$ (0.53)
Diluted net income (loss) per share (in dollars per share)	$ 3.57	$ (0.71)	$ (0.53)