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Investment Securities - (Unrealized Losses and Fair Value of Securities) (Details) $ in Thousands	Sep. 30, 2021 USD ($) security	Sep. 30, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	31	51
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (1,584)	$ (5,833)
12 months or more, unrealized gross losses	(1,056)	(3,781)
Total, unrealized gross losses	(2,640)	(9,614)
Less than 12 months, fair value	52,397	675,518
12 months or more, fair value	102,324	285,161
Total, fair value	154,721	960,679
Held-to-maturity securities
Less than 12 months, unrealized gross losses	0	(52)
12 months or more, unrealized gross losses	0	0
Total unrealized gross losses	0	(52)
Less than 12 months, fair value	0	6,853
12 months or more, fair value	0	0
Total, fair value	0	6,853
Less than 12 months, unrealized gross losses	(1,584)	(5,885)
12 months or more, unrealized gross losses	(1,056)	(3,781)
Total unrealized gross losses	(2,640)	(9,666)
Less than 12 months, fair value	52,397	682,371
12 months or more, fair value	102,324	285,161
Total fair value	154,721	967,532
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses		(74)
12 months or more, unrealized gross losses		(405)
Total, unrealized gross losses		(479)
Less than 12 months, fair value		45,875
12 months or more, fair value		24,596
Total, fair value		70,471
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(15)	(5,481)
12 months or more, unrealized gross losses	(614)	(3,027)
Total, unrealized gross losses	(629)	(8,508)
Less than 12 months, fair value	4,639	587,746
12 months or more, fair value	70,058	204,369
Total, fair value	74,697	792,115
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(1,569)	(278)
12 months or more, unrealized gross losses	(442)	(349)
Total, unrealized gross losses	(2,011)	(627)
Less than 12 months, fair value	47,758	41,897
12 months or more, fair value	32,266	56,196
Total, fair value	$ 80,024	$ 98,093

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