Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Receivables [Abstract]
Average Balance, Private Education Loans	$ 15,687,803	$ 12,217,890	$ 15,569,337	$ 12,017,799
Average Balance FFELP Loans	980,478	1,076,419	991,740	1,089,836
Average Balance of Personal Loans	60,910	0	48,894	0
Average balance, Total portfolio	$ 16,729,191	$ 13,294,309	$ 16,609,971	$ 13,107,635
Weighted Average Interest Rate, Private Education Loans	8.33%	7.98%	8.30%	8.00%
Weighted Average Interest Rate FFELP loans	3.87%	3.48%	3.78%	3.45%
Weighted Average Interest Rate of Personal Loans	9.28%	0.00%	9.19%