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Fair Value Measurements - Recurring and Nonrecurring (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | $ 1,255,401 |  | $ 1,255,401 |  | $ 1,266,209
Total financial assets | 2,865,435 |  | 2,865,435 |  | 2,902,122
Recurring |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 1,255,401 |  | 1,255,401 |  | 1,266,209
Total financial assets | 1,287,740 |  | 1,287,740 |  | 1,267,348
Total financial liabilities | 10,141 |  | 10,141 |  | 3,006
Recurring | Interest rate contracts |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 10,141 |  | 10,141 |  | 1,139
Derivative liabilities | 10,141 |  | 10,141 |  | 1,139
Recurring | Commercial loan hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 3,005 |  | 3,005 |  |
Derivative liabilities |  |  |  |  | 174
Recurring | Borrowings hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 19,193 |  | 19,193 |  |
Derivative liabilities |  |  |  |  | 1,693
Recurring | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 491 |  | 491 |  | 522
Recurring | U.S. government and agency securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 216,009 |  | 216,009 |  | 211,077
Recurring | Municipal bonds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 22,755 |  | 22,755 |  | 26,624
Recurring | Corporate debt securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 184,232 |  | 184,232 |  | 185,298
Recurring | Agency pass-through certificates |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 828,449 |  | 828,449 |  | 834,297
Recurring | Commercial MBS |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 3,465 |  | 3,465 |  | 8,391
Recurring | Level 1 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 491 |  | 491 |  | 522
Total financial assets | 491 |  | 491 |  | 522
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 |  | 0 |  |
Derivative liabilities |  |  |  |  | 0
Recurring | Level 1 | Borrowings hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 0 |  | 0 |  |
Derivative liabilities |  |  |  |  | 0
Recurring | Level 1 | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 491 |  | 491 |  | 522
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 |  | 0 |  | 0
Recurring | Level 2 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 1,254,910 |  | 1,254,910 |  | 1,265,687
Total financial assets | 1,287,249 |  | 1,287,249 |  | 1,266,826
Total financial liabilities | 10,141 |  | 10,141 |  | 3,006
Recurring | Level 2 | Interest rate contracts |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 10,141 |  | 10,141 |  | 1,139
Derivative liabilities | 10,141 |  | 10,141 |  | 1,139
Recurring | Level 2 | Commercial loan hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 3,005 |  | 3,005 |  |
Derivative liabilities |  |  |  |  | 174
Recurring | Level 2 | Borrowings hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 19,193 |  | 19,193 |  |
Derivative liabilities |  |  |  |  | 1,693
Recurring | Level 2 | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 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 | 216,009 |  | 216,009 |  | 211,077
Recurring | Level 2 | Municipal bonds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 22,755 |  | 22,755 |  | 26,624
Recurring | Level 2 | Corporate debt securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 184,232 |  | 184,232 |  | 185,298
Recurring | Level 2 | Agency pass-through certificates |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 828,449 |  | 828,449 |  | 834,297
Recurring | Level 2 | Commercial MBS |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 3,465 |  | 3,465 |  | 8,391
Recurring | Level 3 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 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 |  | 0 |  |
Derivative liabilities |  |  |  |  | 0
Recurring | Level 3 | Borrowings hedges |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets | 0 |  | 0 |  |
Derivative liabilities |  |  |  |  | 0
Recurring | Level 3 | Equity securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  | 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 |  | 0 |  | $ 0
Nonrecurring |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 21,479 | $ 17,584 | 21,479 | $ 17,584 |
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 | (3,217) | (772) | (4,750) | (2,753) |
Nonrecurring | Impaired loans |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 15,125 | 7,807 | 15,125 | 7,807 |
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 | (3,120) | (305) | (4,094) | (1,666) |
Nonrecurring | Real estate owned |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 6,354 | 9,777 | 6,354 | 9,777 |
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 | (97) | (467) | (656) | (1,087) |
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 | 21,479 | 17,584 | 21,479 | 17,584 |
Nonrecurring | Level 3 | Impaired loans |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | 15,125 | 7,807 | 15,125 | 7,807 |
Nonrecurring | Level 3 | Real estate owned |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value disclosure | $ 6,354 | $ 9,777 | $ 6,354 | $ 9,777 |

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