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Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Sep. 30, 2021
Amortized Cost, Available-for-sale securities
Amortized cost	$ 2,162,737	$ 2,090,070
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	1,308	50,829
Gross unrealized loss	(113,008)	(2,640)
Fair Value, Available-for-sale securities
Available-for-sale securities, at fair value	$ 2,051,037	$ 2,138,259
Yield, Available-for-sale securities
Yield	3.36%	1.69%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 463,299	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains	22	13,522
Gross unrealized losses	(56,461)	0
Fair Value, Held-to-maturity securities
Fair value	$ 406,860	$ 379,547
Yield, Held-to-maturity securities
Without single maturity date		3.17%
Yield	2.88%
Investments	$ 2,626,036	$ 2,456,095
Gross unrealized gains on investments	1,330	64,351
Gross unrealized losses on investments	(169,469)	(2,640)
Total Financial Assets	$ 2,457,897	$ 2,517,806
Yield on investments	3.28%	1.89%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 40,403	$ 47,339
5 to 10 years		14,064
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	240
Gross unrealized gains, 5 to 10 years		136
Gross unrealized losses, 1 to 5 years	(1,049)	0
Gross unrealized losses, 5 to 10 years		0
Fair Value, Available-for-sale securities
1 to 5 years	$ 39,354	47,579
5 to 10 years		$ 14,200
Yield, Available-for-sale securities
1 to 5 years	3.03%	0.44%
5 to 10 years		2.05%
Asset-backed securities
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 22,527	$ 19,730
5 to 10 years	82,962	71,207
Over 10 years	668,482	973,892
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, 5 to 10 years	4	412
Gross unrealized gains, over 10 years	783	14,069
Gross unrealized losses, 1 to 5 years	(1,141)	(434)
Gross unrealized losses, 5 to 10 years	(547)	(97)
Gross unrealized losses, over 10 years	(6,069)	(98)
Fair Value, Available-for-sale securities
1 to 5 years	21,386	19,296
5 to 10 years	82,419	71,522
Over 10 years	$ 663,196	$ 987,863
Yield, Available-for-sale securities
1 to 5 years	3.27%	0.52%
5 to 10 years	3.53%	0.57%
Over 10 years	3.86%	0.94%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Within 1 year	$ 75,000	$ 225,928
1 to 5 years	151,411	86,802
5 to 10 years	114,414	28,804
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	4	7,860
Gross unrealized gains, 1 to 5 years	0	1,345
Gross unrealized gains, 5 to 10 years	0	249
Gross unrealized losses, within 1 year	(200)	0
Gross unrealized losses, 1 to 5 years	(2,748)	0
Gross unrealized losses, 5 to 10 years	(24,124)	0
Fair Value, Available-for-sale securities
Within 1 year	74,804	233,788
1 to 5 years	148,663	88,147
5 to 10 years	$ 90,290	$ 29,053
Yield, Available-for-sale securities
Within 1 year	3.74%	1.57%
1 to 5 years	3.59%	4.64%
5 to 10 years	3.87%	1.95%
Municipal bonds
Amortized Cost, Available-for-sale securities
Within 1 year		$ 1,493
5 to 10 years	$ 5,751	5,781
Over 10 years	29,871	29,909
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year		17
Gross unrealized gains, 5 to 10 years	0	294
Gross unrealized gains, over 10 years	400	2,490
Gross unrealized losses, within 1 year		0
Gross unrealized losses, 5 to 10 years	(361)	0
Gross unrealized losses, over 10 years	(699)	0
Fair Value, Available-for-sale securities
Within 1 year		1,510
5 to 10 years	5,390	6,075
Over 10 years	$ 29,572	$ 32,399
Yield, Available-for-sale securities
Within 1 year		0.00%
5 to 10 years	3.00%	0.22%
Over 10 years	5.85%	5.85%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 971,916	$ 585,121
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	117	23,717
Gross unrealized losses, without single maturity date	(76,070)	(2,011)
Fair Value, Available-for-sale securities
Without single maturity date	$ 895,963	$ 606,827
Yield, Available-for-sale securities
Without single maturity date	2.81%	2.60%
Amortized Cost, Held-to-maturity securities
Without single maturity date	$ 463,299	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains, without single maturity date	22	13,522
Gross unrealized losses, without single maturity date	(56,461)	0
Fair Value, Held-to-maturity securities
Without single maturity date	$ 406,860	$ 379,547
Yield, Held-to-maturity securities
Without single maturity date	2.88%	3.17%

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Amount, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity) without single maturity date and not allocated over maturity grouping.

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Sum of the carrying amounts as of the balance sheet date of all investments.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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