Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Average balance of Private Education Loans	$ 19,282,500	$ 16,176,351	$ 12,747,756
Average Balance, FFELP Loans	888,301	970,738	1,063,325
Average Balance of Personal Loans	900,152	112,644	710
Average Balance, Total portfolio	$ 21,070,953	$ 17,259,733	$ 13,811,791
Weighted Average Interest Rate, Private Education Loans	9.10%	8.43%	8.02%
Weighted Average Interest Rate, FFELP Loans	4.57%	3.91%	3.53%
Weighted Average Interest Rate of Personal Loans	11.08%	9.90%	8.53%