Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,254,774	$ 17,275,377	$ 20,617,890	$ 16,833,926
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 19,295,318	$ 16,228,751	$ 18,908,929	$ 15,791,557
Weighted Average Interest Rate	9.16%	8.50%	9.01%	8.37%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 877,279	$ 960,185	$ 898,208	$ 981,106
Weighted Average Interest Rate	4.65%	4.02%	4.47%	3.86%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 1,082,177	$ 86,441	$ 810,753	$ 61,263
Weighted Average Interest Rate	11.03%	9.66%	10.82%	9.44%