Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	$ 1,913,754	$ 2,819,885
Unrealized loss position for less than 12 months, Unrealized Loss	(35,961)	(96,252)
Unrealized loss position for more than 12 months, Fair Value	1,720,600	1,254,553
Unrealized loss position for more than 12 months, Unrealized Loss	$ (114,789)	$ (98,280)
Investment securities:
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Number of securities in loss position \| security	121	174
Unrealized loss position for more than 12 months, Number of securities in loss position \| security	87	51
Floating Interest Rate \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	$ 0	$ 0
Unrealized loss position for less than 12 months, Unrealized Loss	0	0
Unrealized loss position for more than 12 months, Fair Value	19,031	19,027
Unrealized loss position for more than 12 months, Unrealized Loss	(640)	(640)
Floating Interest Rate \| Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	1,224,206	1,884,146
Unrealized loss position for less than 12 months, Unrealized Loss	(12,915)	(36,976)
Unrealized loss position for more than 12 months, Fair Value	586,996	193,964
Unrealized loss position for more than 12 months, Unrealized Loss	(11,527)	(5,934)
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	527,300	621,215
Unrealized loss position for less than 12 months, Unrealized Loss	(22,689)	(56,434)
Unrealized loss position for more than 12 months, Fair Value	476,963	336,782
Unrealized loss position for more than 12 months, Unrealized Loss	(90,402)	(74,403)
Fixed Interest Rate \| Fixed rate U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	162,248	314,524
Unrealized loss position for less than 12 months, Unrealized Loss	(357)	(2,842)
Unrealized loss position for more than 12 months, Fair Value	637,610	704,780
Unrealized loss position for more than 12 months, Unrealized Loss	$ (12,220)	$ (17,303)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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