Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Receivables [Abstract]
Average Balance, Private Education Loans	$ 9,869,025	$ 7,407,774	$ 9,563,290	$ 7,394,985
Average Balance FFELP Loans	1,161,288	1,339,748	1,196,491	1,373,945
Average balance, total portfolio	$ 11,030,313	$ 8,747,522	$ 10,759,781	$ 8,768,930
Weighted Average Interest Rate, Private Education Loans	7.87%	8.20%	7.96%	8.19%
Weighted Average Interest Rate FFELP loans	3.27%	3.23%	3.22%	3.25%