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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, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Interest income on loans receivable	$ 132,757	$ 138,549	$ 266,428	$ 280,695
Interest expense on FHLB advances	(11,991)	(13,368)	(25,189)	(27,026)
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	(1,563)	(112)	(3,104)	(186)
Recognized on derivatives	17,491	(24,676)	22,078	(19,984)
Recognized on hedged items	(17,377)	24,666	(21,833)	20,073
Net income/(expense) recognized on fair value hedges	(1,449)	(122)	(2,859)	(97)
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	3,066	498	6,664	284
Reclassification adjustment of net (gain)/loss included in net income	14,110	0	14,110	0
Net income/(expense) recognized on cash flow hedges	$ 17,176	$ 498	$ 20,774	$ 284

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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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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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