Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	$ 656,616	$ 537,162
Unrealized loss position for less than 12 months, Unrealized Loss	(2,242)	(636)
Unrealized loss position for more than 12 months, Fair Value	48,711	349,825
Unrealized loss position for more than 12 months, Unrealized Loss	$ (578)	$ (2,944)
Investment securities:
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Number of securities in loss position \| security	21	27
Unrealized loss position for more than 12 months, Number of securities in loss position \| security	24	62
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	19,500	19,171
Unrealized loss position for more than 12 months, Unrealized Loss	(148)	(493)
Floating Interest Rate \| Floating rate asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value		0
Unrealized loss position for less than 12 months, Unrealized Loss		0
Unrealized loss position for more than 12 months, Fair Value		6,231
Unrealized loss position for more than 12 months, Unrealized Loss		(1)
Floating Interest Rate \| Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	155,001	172,842
Unrealized loss position for less than 12 months, Unrealized Loss	(1,823)	(593)
Unrealized loss position for more than 12 months, Fair Value	29,211	324,423
Unrealized loss position for more than 12 months, Unrealized Loss	(430)	(2,450)
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	28,706
Unrealized loss position for less than 12 months, Unrealized Loss	(85)
Unrealized loss position for more than 12 months, Fair Value	0
Unrealized loss position for more than 12 months, Unrealized Loss	0
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	472,909	364,320
Unrealized loss position for less than 12 months, Unrealized Loss	(334)	(43)
Unrealized loss position for more than 12 months, Fair Value	0	0
Unrealized loss position for more than 12 months, Unrealized Loss	$ 0	$ 0

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.

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-Name Accounting Standards Codification
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-Section 50
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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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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 less than 12 months, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

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