Securities Available for Sale - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	$ 1,308,445	$ 1,323,573
Gross Unrealized Gains	11,027	8,483
Gross Unrealized Losses	(2,086)	(4,729)
Estimated Fair Value	1,317,386	1,327,327
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	227,007	250,986
Gross Unrealized Gains	3,973	3,811
Gross Unrealized Losses	0	0
Estimated Fair Value	230,980	254,797
U.S. Agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	0	74,412
Gross Unrealized Gains	0	643
Gross Unrealized Losses	0	0
Estimated Fair Value	0	75,055
Mortgage backed-securities \| Government National Mortgage Association
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	664,529	842,460
Gross Unrealized Gains	3,172	3,663
Gross Unrealized Losses	(1,922)	(4,503)
Estimated Fair Value	665,779	841,620
Mortgage backed-securities \| Federal National Mortgage Association
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	212,452	46,449
Gross Unrealized Gains	1,324	96
Gross Unrealized Losses	0	0
Estimated Fair Value	213,776	46,545
Mortgage backed-securities \| Small Business Assistance Program
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	142,921	101,415
Gross Unrealized Gains	2,362	233
Gross Unrealized Losses	0	(213)
Estimated Fair Value	145,283	101,435
States and political subdivision securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	55,525	1,849
Gross Unrealized Gains	123	1
Gross Unrealized Losses	(164)	0
Estimated Fair Value	55,484	1,850
Corporate debt securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	4,998	4,996
Gross Unrealized Gains	24	0
Gross Unrealized Losses	0	(13)
Estimated Fair Value	5,022	4,983
Other
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Amortized Cost	1,013	1,006
Gross Unrealized Gains	49	36
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 1,062	$ 1,042

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)
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Reference 2: 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

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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.

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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
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-Name Accounting Standards Codification
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