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Summary of Significant Accounting Policies - Intangible Assets (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2017 | Sep. 30, 2016
Goodwill [Roll Forward] |  |  |
Goodwill, beginning of period |  | $ 291,503 | $ 291,503
Goodwill, additions |  | 1,650 |
Goodwill, end of period | $ 293,153 | 293,153 | 291,503
Finite-lived Intangible Assets [Roll Forward] |  |  |
Intangible assets. amortization |  | (1,677) | (2,369)
Goodwill and Intangible Assets [Roll Forward] |  |  |
Goodwill and intangible assets, beginning of period |  | 296,989 | 299,358
Goodwill and intangible assets, additions |  | 3,370 |
Goodwill and intangible assets, end of period | 298,682 | 298,682 | 296,989
Core Deposits and Other Intangibles |  |  |
Finite-lived Intangible Assets [Roll Forward] |  |  |
Intangible assets, beginning of period |  | 5,486 | 7,855
Intangible assets, additions |  | 1,720 |
Intangible assets. amortization |  | (1,677) | (2,369)
Intangible assets, end of period | 5,529 | 5,529 | $ 5,486
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] |  |  |
2018 | 1,376 | 1,376 |
2019 | 1,324 | 1,324 |
2020 | 1,295 | 1,295 |
2021 | 457 | 457 |
2022 | 138 | $ 138 |
Acquisition of Businesses in Prior Years |  |  |
Goodwill [Line Items] |  |  |
Amortization of intangible assets | $ 1,500 |  |

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