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Fair Value Measurements - Recurring (Details) - USD ($) $ in Thousands | Jun. 30, 2023 | Sep. 30, 2022
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | $ 2,036,233 | $ 2,051,037
Total financial assets | 2,420,545 | 2,457,897
Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 2,036,233 | 2,051,037
Total financial assets | 2,310,074 | 2,337,524
Total financial liabilities | 65,772 | 67,260
Recurring | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 64,869 | 67,260
Derivative liabilities | 65,772 | 67,260
Recurring | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 2,341 | 2,517
Recurring | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 40,707 | 36,765
Recurring | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 165,924 | 179,945
Recurring | U.S. government and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 192,559 | 39,354
Recurring | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 611,784 | 767,001
Recurring | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 35,049 | 34,962
Recurring | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 246,631 | 313,757
Recurring | Agency pass-through certificates |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 950,210 | 895,963
Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Total financial assets | 0 | 0
Total financial liabilities | 0 | 0
Recurring | Level 1 | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 0 | 0
Derivative liabilities | 0 | 0
Recurring | Level 1 | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 0 | 0
Recurring | Level 1 | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 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
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, at fair value | 0 | 0
Recurring | Level 1 | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Recurring | Level 1 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 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, at fair value | 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, at fair value | 0 | 0
Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 2,036,233 | 2,051,037
Total financial assets | 2,310,074 | 2,337,524
Total financial liabilities | 65,772 | 67,260
Recurring | Level 2 | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 64,869 | 67,260
Derivative liabilities | 65,772 | 67,260
Recurring | Level 2 | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 2,341 | 2,517
Recurring | Level 2 | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 40,707 | 36,765
Recurring | Level 2 | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 165,924 | 179,945
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, at fair value | 192,559 | 39,354
Recurring | Level 2 | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 611,784 | 767,001
Recurring | Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 35,049 | 34,962
Recurring | Level 2 | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 246,631 | 313,757
Recurring | Level 2 | Agency pass-through certificates |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 950,210 | 895,963
Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 | 0
Total financial assets | 0 | 0
Total financial liabilities | 0 | 0
Recurring | Level 3 | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 0 | 0
Derivative liabilities | 0 | 0
Recurring | Level 3 | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 0 | 0
Recurring | Level 3 | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 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
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, at fair value | 0 | 0
Recurring | Level 3 | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 0 |
Recurring | Level 3 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 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, at fair value | 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, at fair value | $ 0 | $ 0

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