Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 20,478,219	$ 16,729,191	$ 20,294,171	$ 16,609,541
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 18,764,768	$ 15,687,803	$ 18,712,533	$ 15,569,337
Weighted Average Interest Rate	9.03%	8.33%	8.93%	8.30%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 898,095	$ 980,478	$ 908,846	$ 991,740
Weighted Average Interest Rate	4.51%	3.87%	4.38%	3.78%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 815,356	$ 60,910	$ 672,792	$ 48,464
Weighted Average Interest Rate	10.65%	9.28%	10.65%	9.24%