Subordinated Debentures and Subordinated Notes Payable - Junior Subordinated Deferrable Interest Debentures (Details) $ / shares in Units, $ in Thousands								3 Months Ended	12 Months Ended
	Jul. 05, 2007 $ / shares shares	Dec. 07, 2006 $ / shares shares	Mar. 10, 2006 $ / shares shares	Jun. 01, 2005 $ / shares shares	Dec. 17, 2003 $ / shares shares	Sep. 25, 2003 $ / shares shares	Dec. 19, 2002 $ / shares shares	Dec. 31, 2017 shares	Sep. 30, 2020 USD ($) trust	Sep. 30, 2019 USD ($)
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Amount of Debentures eligible for treatment as Tier 1 capital | $									$ 108,832	$ 108,636
Mandatorily Redeemable Preferred Securities
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of trusts | trust									7
Mandatorily Redeemable Preferred Securities | Great Western Statutory Trust IV
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of shares caused to be issued | shares					22,400
Par value of shares issued (in dollars per share)					$ 1,000
Maximum interest payment deferral period					5 years
Term of debt instrument					30 years
Redemption price (in dollars per share)					$ 1,000
Mandatorily Redeemable Preferred Securities | GWB Capital Trust VI
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of shares caused to be issued | shares			30,000
Par value of shares issued (in dollars per share)			$ 1,000
Maximum interest payment deferral period			5 years
Redemption price (in dollars per share)			$ 1,000
Mandatorily Redeemable Preferred Securities | GWB Capital Trust II
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Stated interest rate (as a percent)			9.75%
Mandatorily Redeemable Preferred Securities | Sunstate Bancshares Trust II
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of shares caused to be issued | shares				2,000
Par value of shares issued (in dollars per share)				$ 1,000
Maximum interest payment deferral period				5 years
Redemption price (in dollars per share)				$ 1,000
Redemption period following the occurrence of a Special Event				90 days
Mandatorily Redeemable Preferred Securities | HFB Capital Trust III
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of shares caused to be issued | shares							5,000
Par value of shares issued (in dollars per share)							$ 1,000
Maximum interest payment deferral period							5 years
Redemption price (in dollars per share)							$ 1,000
Redemption period following the occurrence of a Special Event							90 days
Mandatorily Redeemable Preferred Securities | HFB Capital Trust IV
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of shares caused to be issued | shares						7,000
Par value of shares issued (in dollars per share)						$ 1,000
Maximum interest payment deferral period						5 years
Redemption price (in dollars per share)						$ 1,000
Redemption period following the occurrence of a Special Event						90 days
Mandatorily Redeemable Preferred Securities | HFB Capital Trust V
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of shares caused to be issued | shares		10,000
Par value of shares issued (in dollars per share)		$ 1,000
Maximum interest payment deferral period		5 years
Redemption price (in dollars per share)		$ 1,000
Redemption period following the occurrence of a Special Event		90 days
Number of shares redeemed (in shares) | shares								5,000
Mandatorily Redeemable Preferred Securities | HFB Capital Trust VI
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Number of shares caused to be issued | shares	2,000
Par value of shares issued (in dollars per share)	$ 1,000
Maximum interest payment deferral period	5 years
Redemption price (in dollars per share)	$ 1,000
Redemption period following the occurrence of a Special Event	90 days
Mandatorily Redeemable Preferred Securities | London Interbank Offered Rate (LIBOR) | Great Western Statutory Trust IV
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)					2.85%
Mandatorily Redeemable Preferred Securities | London Interbank Offered Rate (LIBOR) | GWB Capital Trust VI
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)			1.48%
Mandatorily Redeemable Preferred Securities | London Interbank Offered Rate (LIBOR) | Sunstate Bancshares Trust II
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)				1.85%
Mandatorily Redeemable Preferred Securities | London Interbank Offered Rate (LIBOR) | HFB Capital Trust III
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)							3.35%
Mandatorily Redeemable Preferred Securities | London Interbank Offered Rate (LIBOR) | HFB Capital Trust IV
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)						3.10%
Mandatorily Redeemable Preferred Securities | London Interbank Offered Rate (LIBOR) | HFB Capital Trust V
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)		1.83%
Mandatorily Redeemable Preferred Securities | London Interbank Offered Rate (LIBOR) | HFB Capital Trust VI
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)	1.65%
Junior subordinated debentures payable to non-consolidated trusts
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Amount of Debentures eligible for treatment as Tier 1 capital | $									$ 73,832	73,697
Common shares held in other assets | $									$ 2,520	$ 2,520
HFB Capital Trust V | Junior subordinated debentures payable to non-consolidated trusts
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Basis spread on variable rate (as a percent)									1.83%	1.83%
Common shares held in other assets | $									$ 310	$ 310