Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Receivables [Abstract]
Average Balance, Private Education Loans	$ 16,228,751	$ 12,881,890	$ 15,791,557	$ 12,307,932
Average Balance FFELP Loans	960,185	1,049,803	981,106	1,076,394
Average Balance of Personal Loans	86,441	0	61,263	0
Average balance, Total portfolio	$ 17,275,377	$ 13,931,693	$ 16,833,926	$ 13,384,326
Weighted Average Interest Rate, Private Education Loans	8.50%	8.00%	8.37%	8.00%
Weighted Average Interest Rate FFELP loans	4.02%	3.52%	3.86%	3.48%
Weighted Average Interest Rate of Personal Loans	9.66%	0.00%	9.44%	0.00%