Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($)	3 Months Ended	9 Months Ended
	Jun. 30, 2024	Jun. 30, 2024	Sep. 30, 2023
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 7,385,202,000	$ 7,385,202,000	$ 1,816,870,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(40,379,000)	(40,379,000)	(48,865,000)
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Portfolio layer method, amortized cost	7,350,681,000	7,350,681,000	1,780,503,000
Portfolio layer method, cumulative adjustment, increase (decrease)	(37,908,000)	(37,908,000)	(45,622,000)
Amortized cost, designated in portfolio hedging relationship	2,670,000,000	2,670,000,000	670,000,000
Hedged asset, discontinued	300,000,000	300,000,000
Hedged asset, discontinued, basis adjustment	1,232,211	1,232,211
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	34,521,000	34,521,000	36,367,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	$ (2,471,000)	$ (2,471,000)	$ (3,243,000)