Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Receivables [Abstract]
Average balance of Private Education Loans	$ 9,819,053	$ 7,563,356	$ 5,996,651
Average Balance, FFELP Loans	1,179,723	1,353,497	1,142,979
Average Balance, Total portfolio	$ 10,998,776	$ 8,916,853	$ 7,139,630
Weighted Average Interest Rate, Private Education Loans	7.93%	8.16%	8.16%
Weighted Average Interest Rate, FFELP Loans	3.26%	3.24%	3.32%