REVENUES - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)
Revenue from Contract with Customer [Abstract]
Number of reportable segments | segment						1
Disaggregation of Revenue [Line Items]
Total revenues	$ 22,931	$ 22,464	$ 24,276	$ 46,740	$ 69,671	$ 94,987	$ 66,437
Interchange and card revenue
Disaggregation of Revenue [Line Items]
Total revenues	6,529	6,757	8,244	15,001	21,530	28,078	25,864
Servicing fees from Partner Bank
Disaggregation of Revenue [Line Items]
Total revenues	11,823	10,579	9,372	19,951	31,774	45,105	22,465
Account fees
Disaggregation of Revenue [Line Items]
Total revenues	2,628	2,641	2,686	5,327	7,955	10,668	11,308
University fees
Disaggregation of Revenue [Line Items]
Total revenues	1,474	1,331	1,324	2,655	4,129	5,693	5,320
Other revenue
Disaggregation of Revenue [Line Items]
Total revenues	$ 477	$ 1,156	$ 2,650	$ 3,806	$ 4,283	5,443	1,480
Revenue recognized at a point in time
Disaggregation of Revenue [Line Items]
Total revenues						90,695	62,357
Revenue recognized at a point in time | Interchange and card revenue
Disaggregation of Revenue [Line Items]
Total revenues						28,078	25,864
Revenue recognized at a point in time | Servicing fees from Partner Bank
Disaggregation of Revenue [Line Items]
Total revenues						45,105	22,465
Revenue recognized at a point in time | Account fees
Disaggregation of Revenue [Line Items]
Total revenues						10,668	11,308
Revenue recognized at a point in time | University fees
Disaggregation of Revenue [Line Items]
Total revenues						1,401	1,240
Revenue recognized at a point in time | Other revenue
Disaggregation of Revenue [Line Items]
Total revenues						5,443	1,480
Revenue recognized over time
Disaggregation of Revenue [Line Items]
Total revenues						4,292	4,080
Revenue recognized over time | University fees
Disaggregation of Revenue [Line Items]
Total revenues						$ 4,292	$ 4,080