Securities Available for Sale - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	$ 1,376,942	$ 1,306,919
Gross Unrealized Gains	1,845	11,027
Gross Unrealized Losses	(10,827)	(2,086)
Estimated Fair Value	1,367,960	1,315,860
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	228,039	227,007
Gross Unrealized Gains	579	3,973
Gross Unrealized Losses	(15)	0
Estimated Fair Value	228,603	230,980
Mortgage backed-securities \| Government National Mortgage Association
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	511,457	664,529
Gross Unrealized Gains	228	3,172
Gross Unrealized Losses	(6,635)	(1,922)
Estimated Fair Value	505,050	665,779
Mortgage backed-securities \| Federal National Mortgage Association
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	339,394	210,933
Gross Unrealized Gains	97	1,324
Gross Unrealized Losses	(2,534)	0
Estimated Fair Value	336,957	212,257
Mortgage backed-securities \| Small Business Assistance Program
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	224,005	142,921
Gross Unrealized Gains	726	2,362
Gross Unrealized Losses	(1,001)	0
Estimated Fair Value	223,730	145,283
States and political subdivision securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	73,041	55,525
Gross Unrealized Gains	187	123
Gross Unrealized Losses	(642)	(164)
Estimated Fair Value	72,586	55,484
Corporate debt securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	0	4,998
Gross Unrealized Gains	0	24
Gross Unrealized Losses	0	0
Estimated Fair Value	0	5,022
Other
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	1,006	1,006
Gross Unrealized Gains	28	49
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 1,034	$ 1,055

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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-Publisher FASB
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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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