Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
Average balance of Private Education Loans	$ 22,426,216	$ 22,225,473	$ 19,282,500
Average Balance, FFELP Loans	757,953	814,198	888,301
Average Balance of Personal Loans	582,552	1,141,503	900,152
Average Balance, Total portfolio	$ 23,766,721	$ 24,181,174	$ 21,070,953
Weighted Average Interest Rate, Private Education Loans	8.42%	9.32%	9.10%
Weighted Average Interest Rate, FFELP Loans	3.76%	4.79%	4.57%
Weighted Average Interest Rate of Personal Loans	12.43%	12.09%	11.08%