Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,734,559)	$ (1,929,323)	$ (441,912)	$ (441,912)	$ (414,406)	$ (383,997)	$ (341,121)
Loans held for investment, net	21,709,771			24,667,792
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans	22,614,327			23,189,591
Deferred origination costs and unamortized premium/(discount)	70,406			81,224
Allowance for credit losses	(1,728,811)		(374,300)	(374,300)
Loans held for investment, net	20,955,922			22,896,515
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans	10,353,809			9,830,301
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans	12,260,518			13,359,290
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans	745,556			783,306
Deferred origination costs and unamortized premium/(discount)	2,027			2,143
Allowance for credit losses	(4,363)		(1,633)	(1,633)
Loans held for investment, net	743,220			783,816
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans	0			1,049,007
Deferred origination costs and unamortized premium/(discount)	0			513
Allowance for credit losses	0		(65,877)	(65,877)
Loans held for investment, net	0			983,643
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans	11,540			3,884
Deferred origination costs and unamortized premium/(discount)	474			36
Allowance for credit losses	(1,385)		$ (102)	(102)
Loans held for investment, net	$ 10,629			$ 3,818