Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Average balance of Private Education Loans	$ 16,176,351	$ 12,747,756	$ 9,819,053
Average Balance, FFELP Loans	970,738	1,063,325	1,179,723
Average Balance of Personal Loans	112,644	710	0
Average Balance, Total portfolio	$ 17,259,733	$ 13,811,791	$ 10,998,776
Weighted Average Interest Rate, Private Education Loans	8.43%	8.02%	7.93%
Weighted Average Interest Rate, FFELP Loans	3.91%	3.53%	3.26%
Weighted Average Interest Rate of Personal Loans	9.90%	8.53%	0.00%