Fair Value Measurements - Schedule of Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Sep. 30, 2025
Financial Assets
Available-for-sale securities, at fair value	$ 4,352,258	$ 3,533,201
Total financial assets	5,064,375	4,145,940
Recurring
Financial Assets
Available-for-sale securities, at fair value	4,352,258	3,533,201
Total financial assets	4,495,029	3,684,472
Financial Liabilities
Total financial liabilities	54,160	73,330
Recurring \| Interest rate contracts
Financial Assets
Derivative assets	34,461	37,347
Financial Liabilities
Derivative liabilities	34,527	37,818
Recurring \| Commercial loan fair value hedges
Financial Assets
Derivative assets		1,611
Recurring \| Mortgage loan fair value hedges
Financial Assets
Derivative assets	10,638	13,082
Financial Liabilities
Derivative liabilities	11,165	20,426
Recurring \| Mortgage backed securities fair value hedges
Financial Assets
Derivative assets	294
Financial Liabilities
Derivative liabilities	8,468	15,086
Recurring \| Borrowings cash flow hedges
Financial Assets
Derivative assets	97,378	99,231
Recurring \| U.S. government and agency securities
Financial Assets
Available-for-sale securities, at fair value	202,727	235,919
Recurring \| Asset-backed securities
Financial Assets
Available-for-sale securities, at fair value	474,365	506,334
Recurring \| Municipal bonds
Financial Assets
Available-for-sale securities, at fair value	34,743	35,258
Recurring \| Corporate debt securities
Financial Assets
Available-for-sale securities, at fair value	161,884	152,537
Recurring \| Agency pass-through certificates
Financial Assets
Available-for-sale securities, at fair value	3,478,539	2,603,153
Level 1 \| Recurring
Financial Assets
Available-for-sale securities, at fair value	0	0
Total financial assets	0	0
Financial Liabilities
Total financial liabilities	0	0
Level 1 \| Recurring \| Interest rate contracts
Financial Assets
Derivative assets	0	0
Financial Liabilities
Derivative liabilities	0	0
Level 1 \| Recurring \| Commercial loan fair value hedges
Financial Assets
Derivative assets		0
Level 1 \| Recurring \| Mortgage loan fair value hedges
Financial Assets
Derivative assets	0	0
Financial Liabilities
Derivative liabilities	0	0
Level 1 \| Recurring \| Mortgage backed securities fair value hedges
Financial Assets
Derivative assets	0
Financial Liabilities
Derivative liabilities	0	0
Level 1 \| Recurring \| Borrowings cash flow hedges
Financial Assets
Derivative assets	0	0
Level 1 \| Recurring \| U.S. government and agency securities
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 1 \| Recurring \| Asset-backed securities
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 1 \| Recurring \| Municipal bonds
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 1 \| Recurring \| Corporate debt securities
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 1 \| Recurring \| Agency pass-through certificates
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 2 \| Recurring
Financial Assets
Available-for-sale securities, at fair value	4,352,258	3,533,201
Total financial assets	4,495,029	3,684,472
Financial Liabilities
Total financial liabilities	54,160	73,330
Level 2 \| Recurring \| Interest rate contracts
Financial Assets
Derivative assets	34,461	37,347
Financial Liabilities
Derivative liabilities	34,527	37,818
Level 2 \| Recurring \| Commercial loan fair value hedges
Financial Assets
Derivative assets		1,611
Level 2 \| Recurring \| Mortgage loan fair value hedges
Financial Assets
Derivative assets	10,638	13,082
Financial Liabilities
Derivative liabilities	11,165	20,426
Level 2 \| Recurring \| Mortgage backed securities fair value hedges
Financial Assets
Derivative assets	294
Financial Liabilities
Derivative liabilities	8,468	15,086
Level 2 \| Recurring \| Borrowings cash flow hedges
Financial Assets
Derivative assets	97,378	99,231
Level 2 \| Recurring \| U.S. government and agency securities
Financial Assets
Available-for-sale securities, at fair value	202,727	235,919
Level 2 \| Recurring \| Asset-backed securities
Financial Assets
Available-for-sale securities, at fair value	474,365	506,334
Level 2 \| Recurring \| Municipal bonds
Financial Assets
Available-for-sale securities, at fair value	34,743	35,258
Level 2 \| Recurring \| Corporate debt securities
Financial Assets
Available-for-sale securities, at fair value	161,884	152,537
Level 2 \| Recurring \| Agency pass-through certificates
Financial Assets
Available-for-sale securities, at fair value	3,478,539	2,603,153
Level 3 \| Recurring
Financial Assets
Available-for-sale securities, at fair value	0	0
Total financial assets	0	0
Financial Liabilities
Total financial liabilities	0	0
Level 3 \| Recurring \| Interest rate contracts
Financial Assets
Derivative assets	0	0
Financial Liabilities
Derivative liabilities	0	0
Level 3 \| Recurring \| Commercial loan fair value hedges
Financial Assets
Derivative assets		0
Level 3 \| Recurring \| Mortgage loan fair value hedges
Financial Assets
Derivative assets	0	0
Financial Liabilities
Derivative liabilities	0	0
Level 3 \| Recurring \| Mortgage backed securities fair value hedges
Financial Assets
Derivative assets	0
Financial Liabilities
Derivative liabilities	0	0
Level 3 \| Recurring \| Borrowings cash flow hedges
Financial Assets
Derivative assets	0	0
Level 3 \| Recurring \| U.S. government and agency securities
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 3 \| Recurring \| Asset-backed securities
Financial Assets
Available-for-sale securities, at fair value	0
Level 3 \| Recurring \| Municipal bonds
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 3 \| Recurring \| Corporate debt securities
Financial Assets
Available-for-sale securities, at fair value	0	0
Level 3 \| Recurring \| Agency pass-through certificates
Financial Assets
Available-for-sale securities, at fair value	$ 0	$ 0

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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