Derivatives and Hedging Activities - Schedule of Fair Value, Notional Amount and Balance Sheet Classification (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Sep. 30, 2025
Other assets
Derivatives, Fair Value [Line Items]
Notional amount	$ 2,985,810	$ 2,381,358
Derivative assets, fair value	142,771	151,271
Other assets \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Notional amount	875,810	977,017
Derivative assets, fair value	34,461	37,347
Other assets \| Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Notional amount		34,341
Derivative assets, fair value		1,611
Other assets \| Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Notional amount	670,000	470,000
Derivative assets, fair value	10,638	13,082
Other assets \| Mortgage backed securities fair value hedges
Derivatives, Fair Value [Line Items]
Notional amount	90,000	0
Derivative assets, fair value	294	0
Other assets \| Borrowings cash flow hedges
Derivatives, Fair Value [Line Items]
Notional amount	1,350,000	900,000
Derivative assets, fair value	97,378	99,231
Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	2,095,810	2,687,017
Derivative liabilities, fair value	54,160	73,330
Other liabilities \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Notional amount	875,810	977,017
Derivative liabilities, fair value	34,527	37,818
Other liabilities \| Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Notional amount		0
Derivative liabilities, fair value		0
Other liabilities \| Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Notional amount	700,000	1,100,000
Derivative liabilities, fair value	11,165	20,426
Other liabilities \| Mortgage backed securities fair value hedges
Derivatives, Fair Value [Line Items]
Notional amount	520,000	610,000
Derivative liabilities, fair value	8,468	15,086
Other liabilities \| Borrowings cash flow hedges
Derivatives, Fair Value [Line Items]
Notional amount	0	0
Derivative liabilities, fair value	$ 0	$ 0