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Fair Value Measurements - Fair Value Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Sep. 30, 2019
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | $ 2,249,492 | $ 1,485,742
Total Financial Assets | 2,976,860 | 2,948,837
Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 2,249,492 | 1,485,742
Total Financial Assets | 2,297,693 | 1,507,731
Total Financial Liabilities | 90,129 | 32,546
Recurring | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 48,201 | 20,381
Derivative liabilities | 48,201 | 20,381
Recurring | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative liabilities | 8,492 | 4,288
Recurring | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets |  | 1,608
Derivative liabilities | 16,061 |
Recurring | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative liabilities | 17,375 | 7,877
Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 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 liabilities | 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
Derivative liabilities | 0 |
Recurring | Level 1 | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative liabilities | 0 | 0
Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 2,249,492 | 1,485,742
Total Financial Assets | 2,297,693 | 1,507,731
Total Financial Liabilities | 90,129 | 32,546
Recurring | Level 2 | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 48,201 | 20,381
Derivative liabilities | 48,201 | 20,381
Recurring | Level 2 | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative liabilities | 8,492 | 4,288
Recurring | Level 2 | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets |  | 1,608
Derivative liabilities | 16,061 |
Recurring | Level 2 | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative liabilities | 17,375 | 7,877
Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 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 liabilities | 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
Derivative liabilities | 0 |
Recurring | Level 3 | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative liabilities | 0 | 0
U.S. government and agency securities | Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 18,824 | 21,088
U.S. government and agency securities | Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
U.S. government and agency securities | Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 18,824 | 21,088
U.S. government and agency securities | Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
Asset-backed securities | Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 936,917 | 249,690
Asset-backed securities | Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
Asset-backed securities | Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 936,917 | 249,690
Asset-backed securities | Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities |  | 0
Municipal bonds | Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 38,315 | 22,642
Municipal bonds | Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
Municipal bonds | Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 38,315 | 22,642
Municipal bonds | Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
Corporate debt securities | Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 287,184 | 209,763
Corporate debt securities | Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
Corporate debt securities | Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 287,184 | 209,763
Corporate debt securities | Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
Agency pass-through certificates | Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 968,252 | 982,559
Agency pass-through certificates | Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 0 | 0
Agency pass-through certificates | Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 968,252 | 982,559
Agency pass-through certificates | Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | $ 0 | $ 0

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