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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) item	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 695,700	$ 695,700		$ 471,961
Gross Unrealized Gains	21,205	21,205		26,735
Gross Unrealized Losses	(1,717)	(1,717)		(2,518)
Debt Securities, Available-for-sale, Total	715,188	715,188		496,178
Accrued interest receivable	2,800	2,800		2,700
Other disclosures
Purchases		352,236	$ 14,681
Proceeds from sales of securities	26,873	35,075	$ 18,006
Total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal	$ 15,600	$ 15,600
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal		10.03
Number of originators | item	1	1
Minimum percentage of securities investment		10.00%
Securities pledged to secure deposits and for other purposes	$ 353,200	$ 353,200		335,800
Reimbursement requests greater than 5 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		85
Percent of insured loan		5
Reimbursement requests greater than 9 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		75
Percent of insured loan		9
US Treasury Securities [Member]
Securities Available-for-Sale
Amortized Cost	4,017	$ 4,017		4,014
Gross Unrealized Gains	53	53		103
Debt Securities, Available-for-sale, Total	4,070	4,070		4,117
US Treasury and Government
Securities Available-for-Sale
Amortized Cost	33,915	33,915		6,811
Gross Unrealized Losses	(340)	(340)		(154)
Debt Securities, Available-for-sale, Total	33,575	33,575		6,657
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost				16,098
Gross Unrealized Gains				1,112
Gross Unrealized Losses				(1)
Debt Securities, Available-for-sale, Total				17,209
US States and Political Subdivisions Debt Securities [Member]
Securities Available-for-Sale
Amortized Cost	222,833	222,833		229,352
Gross Unrealized Gains	16,758	16,758		21,269
Gross Unrealized Losses	(639)	(639)		(1,362)
Debt Securities, Available-for-sale, Total	238,952	238,952		249,259
Corporate Bond Securities [Member]
Securities Available-for-Sale
Amortized Cost	5,000	5,000
Gross Unrealized Losses	(8)	(8)
Debt Securities, Available-for-sale, Total	4,992	4,992
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale
Amortized Cost	163,756	163,756		53,999
Gross Unrealized Gains	1,966	1,966		2,866
Gross Unrealized Losses	(308)	(308)		(280)
Debt Securities, Available-for-sale, Total	165,414	165,414		56,585
Asset-backed Securities [Member]
Securities Available-for-Sale
Amortized Cost	187,825	187,825		130,959
Gross Unrealized Gains	1,646	1,646		1,370
Gross Unrealized Losses	(133)	(133)		(511)
Debt Securities, Available-for-sale, Total	189,338	189,338		131,818
FFEL
Securities Available-for-Sale
Debt Securities, Available-for-sale, Total	154,500	$ 154,500
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		97
Collateralized loan obligations
Securities Available-for-Sale
Amortized Cost	61,085	$ 61,085		30,728
Gross Unrealized Gains	18	18		15
Gross Unrealized Losses	(74)	(74)		(210)
Debt Securities, Available-for-sale, Total	61,029	61,029		$ 30,533
Mortgage Backed Securities, Issued by US Government Agencies [Member]
Securities Available-for-Sale
Amortized Cost	17,269	17,269
Gross Unrealized Gains	764	764
Gross Unrealized Losses	(215)	(215)
Debt Securities, Available-for-sale, Total	17,818	17,818
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost	32,836	32,836
Debt Securities, Available-for-sale, Total	$ 34,339	$ 34,339

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