Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	$ 1,384,085	$ 1,308,445
Gross Unrealized Gains	2,251	11,027
Gross Unrealized Losses	(14,778)	(2,086)
Estimated Fair Value	1,371,558	1,317,386
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	227,265	227,007
Gross Unrealized Gains	1,417	3,973
Gross Unrealized Losses	(15)	0
Estimated Fair Value	228,667	230,980
U.S. Agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost		0
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		0
Mortgage-backed securities \| Government National Mortgage Association
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	633,330	664,529
Gross Unrealized Gains	485	3,172
Gross Unrealized Losses	(8,123)	(1,922)
Estimated Fair Value	625,692	665,779
Mortgage-backed securities \| Federal National Mortgage Association
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	281,741	212,452
Gross Unrealized Gains	0	1,324
Gross Unrealized Losses	(3,179)	0
Estimated Fair Value	278,562	213,776
Mortgage-backed securities \| Small Business Assistance Program
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	167,328	142,921
Gross Unrealized Gains	283	2,362
Gross Unrealized Losses	(1,653)	0
Estimated Fair Value	165,958	145,283
States and political subdivision securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	68,410	55,525
Gross Unrealized Gains	2	123
Gross Unrealized Losses	(1,808)	(164)
Estimated Fair Value	66,604	55,484
Corporate debt securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	4,998	4,998
Gross Unrealized Gains	24	24
Gross Unrealized Losses	0	0
Estimated Fair Value	5,022	5,022
Other
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	1,013	1,013
Gross Unrealized Gains	40	49
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 1,053	$ 1,062

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=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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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.

+ References

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

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-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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