Derivatives and Hedging Activities - Summary of Fair Value, Notional Amount and Balance Sheet Classification (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Sep. 30, 2023
Other assets
Derivatives, Fair Value [Line Items]
Notional amount	$ 4,685,245	$ 2,516,405
Derivative assets, fair value	276,484	312,971
Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	978,203	806,744
Derivative liabilities, fair value	64,075	79,668
Client swap program hedges | Other assets
Derivatives, Fair Value [Line Items]
Notional amount	978,203	806,744
Derivative assets, fair value	63,374	78,797
Client swap program hedges | Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	978,203	806,744
Derivative liabilities, fair value	64,075	79,668
Commercial loan fair value hedges | Other assets
Derivatives, Fair Value [Line Items]
Notional amount	37,042	39,661
Derivative assets, fair value	2,534	3,405
Commercial loan fair value hedges | Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	0	0
Derivative liabilities, fair value	0	0
Mortgage loan fair value hedges | Other assets
Derivatives, Fair Value [Line Items]
Notional amount	2,670,000	670,000
Derivative assets, fair value	59,243	46,396
Mortgage loan fair value hedges | Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	0	0
Derivative liabilities, fair value	0	0
Borrowings cash flow hedges | Other assets
Derivatives, Fair Value [Line Items]
Notional amount	1,000,000	1,000,000
Derivative assets, fair value	151,333	184,373
Borrowings cash flow hedges | Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	0	0
Derivative liabilities, fair value	$ 0	$ 0