Subordinated Debentures and Subordinated Notes Payable - Summary of Subordinated Debentures and Notes Payable (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2015	Sep. 30, 2019
Debt Instrument [Line Items]
Total	$ 108,792		$ 108,636
Junior Subordinated Debt
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	75,920		75,920
Total junior subordinated debentures payable, common shares held in other assets	2,520		2,520
Less: fair value adjustment	(2,122)		(2,223)
Total	73,798		73,697
Junior Subordinated Debt | GW Statutory Trust IV, variable rate of 2.85%, plus 3 month LIBOR
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	23,093		23,093
Total junior subordinated debentures payable, common shares held in other assets	$ 693		693
Junior Subordinated Debt | GW Statutory Trust IV, variable rate of 2.85%, plus 3 month LIBOR | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.85%
Junior Subordinated Debt | GW Statutory Trust VI, variable rate of 1.48%, plus 3 month LIBOR
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	$ 30,928		30,928
Total junior subordinated debentures payable, common shares held in other assets	$ 928		928
Junior Subordinated Debt | GW Statutory Trust VI, variable rate of 1.48%, plus 3 month LIBOR | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.48%
Junior Subordinated Debt | SSB Trust II, variable rate of 1.85%, plus 3 month LIBOR
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	$ 2,062		2,062
Total junior subordinated debentures payable, common shares held in other assets	$ 62		62
Junior Subordinated Debt | SSB Trust II, variable rate of 1.85%, plus 3 month LIBOR | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.85%
Junior Subordinated Debt | HF Capital Trust III, variable rate of 3.35%, plus 3 month LIBOR
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	$ 5,155		5,155
Total junior subordinated debentures payable, common shares held in other assets	$ 155		155
Junior Subordinated Debt | HF Capital Trust III, variable rate of 3.35%, plus 3 month LIBOR | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	3.35%
Junior Subordinated Debt | HF Capital Trust IV, variable rate of 3.10%, plus 3 month LIBOR
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	$ 7,217		7,217
Total junior subordinated debentures payable, common shares held in other assets	$ 217		217
Junior Subordinated Debt | HF Capital Trust IV, variable rate of 3.10%, plus 3 month LIBOR | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	3.10%
Junior Subordinated Debt | HF Capital Trust V, variable rate of 1.83%, plus 3 month LIBOR
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	$ 5,310		5,310
Total junior subordinated debentures payable, common shares held in other assets	$ 310		310
Junior Subordinated Debt | HF Capital Trust V, variable rate of 1.83%, plus 3 month LIBOR | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.83%
Junior Subordinated Debt | HF Capital Trust VI, variable rate of 1.65%, plus 3 month LIBOR
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	$ 2,155		2,155
Total junior subordinated debentures payable, common shares held in other assets	$ 155		155
Junior Subordinated Debt | HF Capital Trust VI, variable rate of 1.65%, plus 3 month LIBOR | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.65%
Subordinated Debt
Debt Instrument [Line Items]
Total	$ 34,994		34,939
Less: unamortized debt issuance costs	(6)		(61)
Stated interest rate (as a percent)		4.875%
Subordinated Debt | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)		3.15%
Subordinated Debt | Fixed to floating rate, 4.875% per annum
Debt Instrument [Line Items]
Total subordinated debentures payable, amount outstanding	$ 35,000		$ 35,000
Stated interest rate (as a percent)	4.875%