REVENUES - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) segment	Mar. 31, 2020 USD ($)
Revenue from Contract with Customer [Abstract]
Number of reportable segments | segment	1
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	$ 24,383	$ 15,758
Interchange and card revenue
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	8,351	6,607
Servicing fees from partner bank
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	9,372	4,765
Account fees
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	2,686	2,909
University fees
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	1,324	1,285
Other revenue
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	2,650	192
Revenue recognized at a point in time
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	23,335	14,766
Revenue recognized at a point in time | Interchange and card revenue
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	8,351	6,607
Revenue recognized at a point in time | Servicing fees from partner bank
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	9,372	4,765
Revenue recognized at a point in time | Account fees
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	2,686	2,909
Revenue recognized at a point in time | University fees
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	276	293
Revenue recognized at a point in time | Other revenue
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	2,650	192
Revenue recognized over time
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	1,048	992
Revenue recognized over time | University fees
Disaggregation of Revenue [Line Items]
Total revenue recognized from contracts with customers	$ 1,048	$ 992