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Fair Value Measurements - Recurring and Nonrecurring (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | $ 1,545,606 |  | $ 1,545,606 |  |
Available-for-sale securities |  |  |  |  | $ 1,314,957
Total financial assets | 3,081,158 |  | 3,081,158 |  | 2,863,727
Recurring |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 1,545,606 |  | 1,545,606 |  |
Available-for-sale securities |  |  |  |  | 1,314,957
Total financial assets | 1,554,028 |  | 1,554,028 |  | 1,353,795
Total financial liabilities | 2,916 |  | 2,916 |  | 12,731
Recurring | Interest rate contracts |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 2,822 |  | 2,822 |  | 12,731
Derivative liabilities | 2,822 |  | 2,822 |  | 12,731
Recurring | Commercial loan hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets |  |  |  |  | 3,857
Derivative liabilities | 94 |  | 94 |  |
Recurring | Borrowings cash flow hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 5,600 |  | 5,600 |  | 22,250
Recurring | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | 488
Recurring | U.S. government and agency securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 235,280 |  | 235,280 |  | 207,293
Recurring | Municipal bonds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 22,408 |  | 22,408 |  | 22,978
Recurring | Corporate debt securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 257,981 |  | 257,981 |  | 184,695
Recurring | Agency pass-through certificates |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 1,029,937 |  | 1,029,937 |  | 896,041
Recurring | Commercial MBS |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | 3,462
Recurring | Level 1 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  |
Available-for-sale securities |  |  |  |  | 488
Total financial assets | 0 |  | 0 |  | 488
Total financial liabilities | 0 |  | 0 |  | 0
Recurring | Level 1 | Interest rate contracts |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 0 |  | 0 |  | 0
Derivative liabilities | 0 |  | 0 |  | 0
Recurring | Level 1 | Commercial loan hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets |  |  |  |  | 0
Derivative liabilities | 0 |  | 0 |  |
Recurring | Level 1 | Borrowings cash flow hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 0 |  | 0 |  | 0
Recurring | Level 1 | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | 488
Recurring | Level 1 | U.S. government and agency securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 1 | Municipal bonds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 1 | Corporate debt securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 1 | Agency pass-through certificates |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 1 | Commercial MBS |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | 0
Recurring | Level 2 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 1,545,606 |  | 1,545,606 |  |
Available-for-sale securities |  |  |  |  | 1,314,469
Total financial assets | 1,554,028 |  | 1,554,028 |  | 1,353,307
Total financial liabilities | 2,916 |  | 2,916 |  | 12,731
Recurring | Level 2 | Interest rate contracts |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 2,822 |  | 2,822 |  | 12,731
Derivative liabilities | 2,822 |  | 2,822 |  | 12,731
Recurring | Level 2 | Commercial loan hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets |  |  |  |  | 3,857
Derivative liabilities | 94 |  | 94 |  |
Recurring | Level 2 | Borrowings cash flow hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 5,600 |  | 5,600 |  | 22,250
Recurring | Level 2 | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | 0
Recurring | Level 2 | U.S. government and agency securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 235,280 |  | 235,280 |  | 207,293
Recurring | Level 2 | Municipal bonds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 22,408 |  | 22,408 |  | 22,978
Recurring | Level 2 | Corporate debt securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 257,981 |  | 257,981 |  | 184,695
Recurring | Level 2 | Agency pass-through certificates |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 1,029,937 |  | 1,029,937 |  | 896,041
Recurring | Level 2 | Commercial MBS |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | 3,462
Recurring | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  |
Available-for-sale securities |  |  |  |  | 0
Total financial assets | 0 |  | 0 |  | 0
Total financial liabilities | 0 |  | 0 |  | 0
Recurring | Level 3 | Interest rate contracts |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 0 |  | 0 |  | 0
Derivative liabilities | 0 |  | 0 |  | 0
Recurring | Level 3 | Commercial loan hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets |  |  |  |  | 0
Derivative liabilities | 0 |  | 0 |  |
Recurring | Level 3 | Borrowings cash flow hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 0 |  | 0 |  | 0
Recurring | Level 3 | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | 0
Recurring | Level 3 | U.S. government and agency securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 3 | Municipal bonds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 3 | Corporate debt securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 3 | Agency pass-through certificates |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 0
Recurring | Level 3 | Commercial MBS |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities |  |  |  |  | $ 0
Nonrecurring |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 5,866 | $ 12,863 | 5,866 | $ 12,863 |
Nonrecurring | Changes measurement |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loss related to impaired loans and real estate held for sale, measured on nonrecurring basis | (80) | (845) | (838) | (1,532) |
Nonrecurring | Impaired loans |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 3,316 | 7,343 | 3,316 | 7,343 |
Nonrecurring | Impaired loans | Changes measurement |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loss related to impaired loans and real estate held for sale, measured on nonrecurring basis | (511) | (466) | (1,237) | (973) |
Nonrecurring | Real estate owned |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 2,550 | 5,520 | 2,550 | 5,520 |
Nonrecurring | Real estate owned | Changes measurement |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loss related to impaired loans and real estate held for sale, measured on nonrecurring basis | 431 | (379) | 399 | (559) |
Nonrecurring | Level 1 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 0 | 0 | 0 | 0 |
Nonrecurring | Level 1 | Impaired loans |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 0 | 0 | 0 | 0 |
Nonrecurring | Level 1 | Real estate owned |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 0 | 0 | 0 | 0 |
Nonrecurring | Level 2 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 0 | 0 | 0 | 0 |
Nonrecurring | Level 2 | Impaired loans |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 0 | 0 | 0 | 0 |
Nonrecurring | Level 2 | Real estate owned |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 0 | 0 | 0 | 0 |
Nonrecurring | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 5,866 | 12,863 | 5,866 | 12,863 |
Nonrecurring | Level 3 | Impaired loans |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 3,316 | 7,343 | 3,316 | 7,343 |
Nonrecurring | Level 3 | Real estate owned |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | $ 2,550 | $ 5,520 | $ 2,550 | $ 5,520 |

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