Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Numerator: Basic and Diluted
Net income (loss)	$ 2,130	$ 713	$ (2,068)	$ (22,526)	$ (3,661)	$ (3,967)	$ (1,890)	$ (672)	$ (21,751)	$ (10,190)	$ 5,020
Dividends									(11,027)	(11,009)	(10,988)
Undistributed income (loss) - Basic									(32,778)	(21,199)	(5,968)
Undistributed income (loss) - Diluted									$ (32,778)	$ (21,199)	$ (5,968)
Percentage allocated to common shares									100.00%	100.00%	99.00%
Undistributed income (loss) allocated to common shares - Basic									$ (32,778)	$ (21,199)	$ (5,910)
Undistributed income (loss) allocated to common shares - Diluted									(32,778)	(21,199)	(5,910)
Dividends paid on common shares outstanding									10,933	10,905	10,881
Net income (loss) available to common shares - Basic									(21,845)	(10,294)	4,971
Net income (loss) available to common shares - Diluted									$ (21,845)	$ (10,294)	$ 4,971
Denominator: Basic and Diluted
Weighted average common shares outstanding - Basic									12,419,564	12,397,099	12,361,483
Adjustment for dilutive potential common shares											4,714
Weighted average shares outstanding - Diluted									12,419,564	12,397,099	12,366,197
Basic net income (loss) per share (in dollars per share)	$ 0.17	$ 0.06	$ (0.17)	$ (1.82)	$ (0.30)	$ (0.32)	$ (0.15)	$ (0.06)	$ (1.75)	$ (0.83)	$ 0.40
Diluted net income (loss) per share (in dollars per share)	$ 0.17	$ 0.06	$ (0.17)	$ (1.82)	$ (0.30)	$ (0.32)	$ (0.15)	$ (0.06)	$ (1.75)	$ (0.83)	$ 0.40