Fair Value of Financial Instruments - Narrative (Details) - Level 2 - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2016
Market Approach Valuation Technique | Junior subordinated notes
Financial Instruments Not Measured at Fair Value
Market interest rate (as a percent)	7.11%	6.37%
Market Approach Valuation Technique | Mortgages | Minimum
Financial Instruments Not Measured at Fair Value
Market interest rate (as a percent)	3.85%	3.78%
Market Approach Valuation Technique | Mortgages | Maximum
Financial Instruments Not Measured at Fair Value
Market interest rate (as a percent)	5.30%	5.02%
Carrying and Fair Value | Junior subordinated notes
Financial Instruments Not Measured at Fair Value
Estimated fair value (lower) higher than carrying value	$ (15,243)	$ (15,705)
Carrying and Fair Value | Mortgages
Financial Instruments Not Measured at Fair Value
Estimated fair value (lower) higher than carrying value	$ (13,750)	$ 11,400