Allowance for Loan Losses - Loan Status of Private Education Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ (441,912)	$ (341,121)	$ (251,475)	$ (184,701)
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	5,687,405	5,260,445	4,757,732
Loans in forbearance	714,516	577,164	468,402
Loans current	$ 16,315,651	$ 14,289,705	$ 11,911,128
Loans current, in percentage	97.20%	97.40%	97.60%
Total TDR loans in repayment	$ 16,787,670	$ 14,666,856	$ 12,206,033
Loans in repayment, in percentage	100.00%	100.00%	100.00%
Total Private Education Loans, gross	$ 23,189,591	$ 20,504,465	$ 17,432,167
Private Education Loans deferred origination costs and unamortized premium/(discount)	81,224	68,321	56,378
Total Private Education Loans	23,270,815	20,572,786	17,488,545
Allowance for loan losses	(374,300)	(277,943)	(243,715)
Private Education Loans, net	$ 22,896,515	$ 20,294,843	$ 17,244,830
Percentage of Private Education Loans in repayment	72.40%	71.50%	70.00%
Delinquencies as a percentage of Private Education Loans in repayment	2.80%	2.60%	2.40%
Loans in forbearance as a percentage of Private Education Loans in repayment and forbearance	4.10%	3.80%	3.70%
Consumer Portfolio Segment | Student Loan | Loans delinquent 31-60 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent	$ 288,051	$ 231,216	$ 179,002
Loans delinquent (in percentage)	1.70%	1.60%	1.50%
Consumer Portfolio Segment | Student Loan | Loans delinquent 61-90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent	$ 121,302	$ 95,105	$ 78,292
Loans delinquent (in percentage)	0.70%	0.70%	0.60%
Consumer Portfolio Segment | Student Loan | Loans delinquent greater than 90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent	$ 62,666	$ 50,830	$ 37,611
Loans delinquent (in percentage)	0.40%	0.30%	0.30%