Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Receivables [Abstract]
Average Balance, Private Education Loans	$ 11,817,708	$ 9,454,579
Average Balance FFELP Loans	1,103,253	1,234,682
Average balance, Total portfolio	$ 12,920,961	$ 10,689,261
Weighted Average Interest Rate, Private Education Loans	8.03%	8.07%
Weighted Average Interest Rate FFELP loans	3.42%	3.19%