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Summary of Significant Accounting Policies - Intangible Assets (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2016 | Sep. 30, 2015
Goodwill and Intangible Assets [Roll Forward] |  |
Goodwill, beginning of period | $ 291,503 | $ 291,503
Goodwill, end of period | 291,503 | 291,503
Intangible assets. amortization | (2,369) | (3,551)
Goodwill and intangible assets, beginning of period | 299,358 | 302,909
Goodwill and intangible assets, end of period | 296,989 | 299,358
Core Deposit [Member] |  |
Goodwill and Intangible Assets [Roll Forward] |  |
Intangible assets, beginning of period | 7,855 | 11,406
Intangible assets. amortization | (2,369) | (3,551)
Intangible assets, end of period | 5,486 | $ 7,855
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] |  |
2017 | 1,648 |
2018 | 1,204 |
2019 | 1,157 |
2020 | 1,157 |
2021 | $ 320 |

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- Definition Amount of increase (decrease) in accumulated depreciation, depletion and amortization of property, plant and equipment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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- Definition Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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- Definition Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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