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Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Sep. 30, 2024 | Mar. 31, 2025
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | $ 7,287,540,000 | $ 6,265,535,000
Hedged Liability, Statement of Financial Position [Extensible Enumeration] | Net loans |
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | $ 20,005,000 | 1,087,000
Loans receivable |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items |  | 5,674,948,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items |  | (7,814,000)
Available-for-sale securities, at fair value |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items |  | 590,587,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items |  | 8,901,000
Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 1,981,554,000 | 3,532,069,000
Mortgage loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Portfolio layer method, amortized cost | 7,252,017,000 | 5,642,530,000
Portfolio layer method, cumulative adjustment, increase (decrease) | 21,476,000 | (5,937,000)
Amortized cost, designated in portfolio hedging relationship |  | 1,570,000,000
Hedged asset, discontinued | 300,000,000 |
Hedged asset, discontinued fair value | 1,232,211 |
Mortgage loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 0 | 1,570,000,000
Commercial loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | 35,523,000 | 32,418,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | (1,471,000) | (1,877,000)
Commercial loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | $ 37,042,000 | 34,341,000
Mortgage backed securities fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items |  | 590,587,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items |  | 8,901,000
Amortized cost, designated in portfolio hedging relationship |  | 400,000,000
Mortgage backed securities fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount |  | $ 0

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