Securities Available for Sale - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 1,427,454	$ 1,376,942
Gross Unrealized Gains	39	1,845
Gross Unrealized Losses	(41,843)	(10,827)
Estimated Fair Value	1,385,650	1,367,960
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	168,394	228,039
Gross Unrealized Gains	0	579
Gross Unrealized Losses	(1,222)	(15)
Estimated Fair Value	167,172	228,603
Mortgage backed-securities \| Government National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	442,458	511,457
Gross Unrealized Gains	35	228
Gross Unrealized Losses	(16,335)	(6,635)
Estimated Fair Value	426,158	505,050
Mortgage backed-securities \| Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	297,380	169,147
Gross Unrealized Gains	0	75
Gross Unrealized Losses	(7,055)	(1,247)
Estimated Fair Value	290,325	167,975
Mortgage backed-securities \| Federal National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	188,192	170,247
Gross Unrealized Gains	0	22
Gross Unrealized Losses	(6,081)	(1,287)
Estimated Fair Value	182,111	168,982
Mortgage backed-securities \| Small Business Assistance Program
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	260,458	224,005
Gross Unrealized Gains	0	726
Gross Unrealized Losses	(9,345)	(1,001)
Estimated Fair Value	251,113	223,730
States and political subdivision securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	69,566	73,041
Gross Unrealized Gains	4	187
Gross Unrealized Losses	(1,795)	(642)
Estimated Fair Value	67,775	72,586
Other
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	1,006	1,006
Gross Unrealized Gains	0	28
Gross Unrealized Losses	(10)	0
Estimated Fair Value	$ 996	$ 1,034

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

No definition available.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

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- References

No definition available.

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