Investment Securities - Continuous Unrealized Loss Position of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Securities available for sale:
Less than 12 months, Fair Value	$ 549,632	$ 354,307
Less than 12 months, Gross unrealized (losses)	(12,740)	(5,692)
12 months or longer, Fair value	170,675	64,047
12 months or longer, Gross unrealized (losses)	(9,353)	(2,390)
Total, Fair value	720,307	418,354
Total, Gross unrealized (losses)	(22,093)	(8,082)
U.S. government and agency obligations
Securities available for sale:
Less than 12 months, Fair Value	44,092	14,831
Less than 12 months, Gross unrealized (losses)	(722)	(116)
12 months or longer, Fair value	4,774	0
12 months or longer, Gross unrealized (losses)	(206)	0
Total, Fair value	48,866	14,831
Total, Gross unrealized (losses)	(928)	(116)
Mortgage-backed securities and collateralized mortgage obligations
Securities available for sale:
Less than 12 months, Fair Value	485,994	329,554
Less than 12 months, Gross unrealized (losses)	(11,572)	(5,346)
12 months or longer, Fair value	156,550	49,695
12 months or longer, Gross unrealized (losses)	(8,497)	(1,742)
Total, Fair value	642,544	379,249
Total, Gross unrealized (losses)	(20,069)	(7,088)
Corporate debt securities
Securities available for sale:
Less than 12 months, Fair Value	14,894	9,824
Less than 12 months, Gross unrealized (losses)	(98)	(176)
12 months or longer, Fair value	9,351	9,644
12 months or longer, Gross unrealized (losses)	(650)	(356)
Total, Fair value	24,245	19,468
Total, Gross unrealized (losses)	(748)	(532)
Trust preferred securities
Securities available for sale:
Less than 12 months, Fair Value	4,652	0
Less than 12 months, Gross unrealized (losses)	(348)	0
12 months or longer, Fair value	0	4,708
12 months or longer, Gross unrealized (losses)	0	(292)
Total, Fair value	4,652	4,708
Total, Gross unrealized (losses)	$ (348)	(292)
Equity securities
Securities available for sale:
Less than 12 months, Fair Value		98
Less than 12 months, Gross unrealized (losses)		(54)
12 months or longer, Fair value		0
12 months or longer, Gross unrealized (losses)		0
Total, Fair value		98
Total, Gross unrealized (losses)		$ (54)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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