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, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Interest income/(expense), including the effects of fair value and cash flow hedges	$ 138,549	$ 141,061	$ 280,695	$ 278,126
Interest expense during the year (including swap interest income and expense)	13,368	17,846	27,026	34,737
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	(112)	58	(186)	77
Recognized on derivatives	(24,676)	(1,600)	(19,984)	(3,917)
Recognized on hedged items	24,666	1,602	20,073	3,881
Net income/(expense) recognized on fair value hedges	(122)	60	(97)	41
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	498	(854)	284	(1,404)
Amount of derivative gain/(loss) reclassified from AOCI into interest income/expense	0	0	0	0
Net income/(expense) recognized on cash flow hedges	$ 498	$ (854)	$ 284	$ (1,404)

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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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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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