v3.26.1
Derivatives and Hedging Activities - Schedule of Gains/(Losses) on Derivative Instruments in Fair Value and Cash Flow Accounting Hedging Relationships (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Derivative Instruments, Gain (Loss) [Line Items]        
Interest income/(expense), including the effects of fair value and cash flow hedges $ 267,243 $ 279,476 $ 793,598 $ 848,150
Interest on Mortgage-backed securities 46,569 27,855 129,812 70,118
Interest income/(expense), including the effects of fair value and cash flow hedges (27,708) (16,991) (64,044) (67,753)
Interest rate contracts | Fair value hedging | Location, Statement of Income, Balance [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeases        
Gain/(loss) on fair value hedging relationships:        
Amounts related to interest settlements on derivatives 1,366 3,780 5,735 16,762
Recognized on derivatives 7,736 (10,789) 14,600 6,031
Recognized on hedged items (7,791) 10,800 (14,783) (13,002)
Net income/(expense) recognized on fair value hedges 1,311 3,791 5,552 9,791
Interest rate contracts | Fair value hedging | Location, Statement of Income, Balance [Axis]: us-gaap:InterestIncomeSecuritiesMortgageBacked        
Gain/(loss) on fair value hedging relationships:        
Amounts related to interest settlements on derivatives (500) 314 (832) 430
Recognized on derivatives 6,393 (4,822) 13,305 (13,504)
Recognized on hedged items (6,442) 4,927 (13,431) 13,828
Net income/(expense) recognized on fair value hedges (549) 419 (958) 754
Interest rate contracts | Cash flow hedging | Location, Statement of Income, Balance [Axis]: us-gaap:InterestExpenseBorrowings        
Gain/(loss) on cash flow hedging relationships:        
Amounts related to interest settlements on derivatives 6,841 8,649 22,492 26,702
Amount of derivative gain/(loss) reclassified from AOCI into interest income/expense 0 0 0 0
Net income/(expense) recognized on cash flow hedges $ 6,841 $ 8,649 $ 22,492 $ 26,702