Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Average balance of Private Education Loans	$ 22,225,473	$ 19,282,500	$ 16,176,351
Average Balance, FFELP Loans	814,198	888,301	970,738
Average Balance of Personal Loans	1,141,503	900,152	112,644
Average Balance, Total portfolio	$ 24,181,174	$ 21,070,953	$ 17,259,733
Weighted Average Interest Rate, Private Education Loans	9.32%	9.10%	8.43%
Weighted Average Interest Rate, FFELP Loans	4.79%	4.57%	3.91%
Weighted Average Interest Rate of Personal Loans	12.09%	11.08%	9.90%