Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 1,774,403	$ 1,427,454
Gross Unrealized Gains	7,860	39
Gross Unrealized Losses	(18,958)	(41,843)
Estimated Fair Value	1,763,305	1,385,650
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	93,696	168,394
Gross Unrealized Gains	419	0
Gross Unrealized Losses	(296)	(1,222)
Estimated Fair Value	93,819	167,172
States and political subdivision securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	63,400	69,566
Gross Unrealized Gains	151	4
Gross Unrealized Losses	(681)	(1,795)
Estimated Fair Value	62,870	67,775
Other
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	1,006	1,006
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(6)	(10)
Estimated Fair Value	1,000	996
Government National Mortgage Association \| Mortgage-backed Securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	491,888	442,458
Gross Unrealized Gains	1,811	35
Gross Unrealized Losses	(9,909)	(16,335)
Estimated Fair Value	483,790	426,158
Federal Home Loan Mortgage Corporation \| Mortgage-backed Securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	438,605	297,380
Gross Unrealized Gains	3,697	0
Gross Unrealized Losses	(2,839)	(7,055)
Estimated Fair Value	439,463	290,325
Federal National Mortgage Association \| Mortgage-backed Securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	303,643	188,192
Gross Unrealized Gains	1,268	0
Gross Unrealized Losses	(2,386)	(6,081)
Estimated Fair Value	302,525	182,111
Small Business Assistance Program \| Mortgage-backed Securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	382,165	260,458
Gross Unrealized Gains	514	0
Gross Unrealized Losses	(2,841)	(9,345)
Estimated Fair Value	$ 379,838	$ 251,113

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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