Securities Available for Sale - Schedule of Gross Unrealized Losses on Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Estimated Fair Value	$ 77,331	$ 94,612
12 months or more, Estimated Fair Value	34,258	543,401
Estimated Fair Value	111,589	638,013
12 months or more, Unrealized Losses	(213)	(205)
Less than 12 months, Unrealized Losses	(54)	(4,292)
Unrealized Losses	(267)	(4,497)
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Estimated Fair Value	0	0
12 months or more, Estimated Fair Value	0	44,729
Estimated Fair Value	0	44,729
12 months or more, Unrealized Losses	0	0
Less than 12 months, Unrealized Losses	0	(32)
Unrealized Losses	0	(32)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Estimated Fair Value	77,331	94,612
12 months or more, Estimated Fair Value	34,258	474,979
Estimated Fair Value	111,589	569,591
12 months or more, Unrealized Losses	(213)	(205)
Less than 12 months, Unrealized Losses	(54)	(4,144)
Unrealized Losses	(267)	(4,349)
States and political subdivision securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Estimated Fair Value	0	0
12 months or more, Estimated Fair Value	0	23,693
Estimated Fair Value	0	23,693
12 months or more, Unrealized Losses	0	0
Less than 12 months, Unrealized Losses	0	(116)
Unrealized Losses	0	(116)
Other
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Estimated Fair Value	0	0
12 months or more, Estimated Fair Value	0	0
Estimated Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Less than 12 months, Unrealized Losses	0	0
Unrealized Losses	$ 0	$ 0