Derivatives and Hedging Activities - Gains/(Losses) on Derivative Instruments in Fair Value and Cash Flow Accounting Hedging Relationships (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Derivative Instruments, Gain (Loss) [Line Items]
Interest income on loans receivable	$ 139,260	$ 132,757	$ 277,769	$ 266,428
Interest expense on FHLB advances	(7,525)	(11,991)	(15,368)	(25,189)
Interest rate contracts \| Interest income on loans receivable \| Fair value hedging
Gain/(loss) on fair value hedging relationships:
Amounts related to interest settlements on derivatives	(1,199)	(1,563)	(2,570)	(3,104)
Recognized on derivatives	18,561	17,491	23,061	22,078
Recognized on hedged items	(18,586)	(17,377)	(23,061)	(21,833)
Net income/(expense) recognized on fair value hedges	(1,224)	(1,449)	(2,570)	(2,859)
Interest rate contracts \| Interest expense on FHLB advances \| Cash flow hedging
Gain/(loss) on cash flow hedging relationships:
Amounts related to interest settlements on derivatives	1,715	3,066	3,736	6,664
Reclassification adjustment of net (gain)/loss included in net income	0	14,110	0	14,110
Net income/(expense) recognized on cash flow hedges	$ 1,715	$ 17,176	$ 3,736	$ 20,774

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Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

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Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

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The aggregate interest and fee income generated by: (1) loans the Entity has positive intent and ability to hold for the foreseeable future, or until maturity or payoff, including commercial and consumer loans, whether domestic or foreign, which may consist of: (a) industrial and agricultural; (b) real estate; and (c) real estate construction loans; (d) trade financing; (e) lease financing; (f) home equity lines-of-credit; (g) automobile and other vehicle loans; and (h) credit card and other revolving-type loans and (2) loans and leases held-for-sale which may include mortgage loans, direct financing, and sales-type leases.

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Aggregate amount of interest expense on all borrowings.

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