Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Receivables [Abstract]
Average Balance, Private Education Loans	$ 9,361,711	$ 7,357,599	$ 9,407,888	$ 7,388,484
Average Balance FFELP Loans	1,194,309	1,378,206	1,214,384	1,391,327
Average balance, total portfolio	$ 10,556,020	$ 8,735,805	$ 10,622,272	$ 8,779,811
Weighted Average Interest Rate, Private Education Loans	7.96%	8.22%	8.01%	8.18%
Weighted Average Interest Rate FFELP loans	3.22%	3.32%	3.20%	3.26%