Allowance for Loan Losses - Age Analysis of Past Due Loan Delinquencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
TDR loans in repayment(3) and percentage of each status:
Private Education Loans allowance for losses	$ (112,507)	$ (83,842)	$ (68,081)	$ (69,189)
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	6,869	2,915
Loans in forbearance	43,756	18,620
TDR loans in repayment(3) and percentage of each status:
Loans current	185,936	34,554
Total TDR loans in repayment	$ 215,206	$ 37,867
Loans current, in percentage	86.40%	91.20%
Loans in repayment, in percentage	100.00%	100.00%
Total TDR loans, gross	$ 265,831	$ 59,402
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	3,427,964	3,027,143	2,574,711
Loans in forbearance	241,207	135,018	16,314
TDR loans in repayment(3) and percentage of each status:
Loans current	6,773,095	5,045,600	3,933,143
Total TDR loans in repayment	$ 6,927,266	$ 5,149,215	$ 3,972,317
Loans current, in percentage	97.80%	98.00%	99.00%
Loans in repayment, in percentage	100.00%	100.00%	100.00%
Total TDR loans, gross	$ 10,596,437	$ 8,311,376	$ 6,563,342
Private Education Loans deferred origination costs	27,884	13,845	5,063
Total Private Education Loans	10,624,321	8,325,221	6,568,405
Private Education Loans allowance for losses	(108,816)	(78,574)	(61,763)	$ (65,218)
Percentage of Private Education Loans in repayment(4)	$ 10,515,505	$ 8,246,647	$ 6,506,642
Percentage of Private Education Loans in repayment(4)	65.40%	62.00%	60.50%
Delinquencies as a percentage of Private Education Loans in repayment(4)	2.20%	2.00%	1.00%
Loans in forbearance as a percentage of loans in repayment and forbearance(4)	3.40%	2.60%	0.40%
31 to 60 Days Past Due | Troubled Debt Restructured Loans
TDR loans in repayment(3) and percentage of each status:
Loans delinquent 31-60 days	$ 14,948	$ 1,953
Loans delinquent 31-60 days, in percentage	6.90%	5.20%
31 to 60 Days Past Due | Private Education Loans
TDR loans in repayment(3) and percentage of each status:
Loans delinquent 31-60 days	$ 91,129	$ 63,873	$ 28,854
Loans delinquent 31-60 days, in percentage	1.30%	1.20%	0.70%
61 to 90 Days Past Due | Troubled Debt Restructured Loans
TDR loans in repayment(3) and percentage of each status:
Loans delinquent 31-60 days	$ 9,239	$ 983
Loans delinquent 31-60 days, in percentage	4.30%	2.60%
61 to 90 Days Past Due | Private Education Loans
TDR loans in repayment(3) and percentage of each status:
Loans delinquent 31-60 days	$ 42,048	$ 29,041	$ 10,280
Loans delinquent 31-60 days, in percentage	0.60%	0.60%	0.30%
Equal to Greater than 90 Days Past Due | Troubled Debt Restructured Loans
TDR loans in repayment(3) and percentage of each status:
Loans delinquent 31-60 days	$ 5,083	$ 377
Loans delinquent 31-60 days, in percentage	2.40%	1.00%
Equal to Greater than 90 Days Past Due | Private Education Loans
TDR loans in repayment(3) and percentage of each status:
Loans delinquent 31-60 days	$ 20,994	$ 10,701	$ 40
Loans delinquent 31-60 days, in percentage	0.30%	0.20%	0.00%