Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (144,925)	$ (112,507)	$ (126,249)	$ (91,866)	$ (89,805)	$ (83,842)
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	4,020,242	3,427,964
Loans in forbearance	241,433	241,207
Loans in repayment and percentage of each status:
Loans current	7,860,994	6,773,095
Total Private Education Loans in repayment	8,029,034	6,927,266
Total Private Education loans, gross	12,290,709	10,596,437
Private Education Loans deferred origination costs	35,212	27,884
Total Private Education Loans	12,325,921	10,624,321
Private Education Loans allowance for losses	(142,628)	(108,816)	$ (122,620)	$ (87,310)	$ (85,236)	$ (78,574)
Private Education Loans, net	$ 12,183,293	$ 10,515,505
Loans current, in percentage	97.90%	97.80%
Total Private Education Loans in repayment (as a percentage)	100.00%	100.00%
Percentage of Private Education Loans in repayment (as a percentage)	65.30%	65.40%
Delinquencies as a percentage of Private Education Loans in repayment (as a percentage)	2.10%	2.20%
Loans in forbearance as a percentage of Private Education Loans in repayment and forbearance (as a percentage)	2.90%	3.40%
Loans delinquent 31-60 days | Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 87,990	$ 91,129
Loans delinquent, in percentage	1.10%	1.30%
Loans delinquent 61-90 days | Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 56,377	$ 42,048
Loans delinquent, in percentage	0.70%	0.60%
Loans delinquent greater than 90 days | Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 23,673	$ 20,994
Loans delinquent, in percentage	0.30%	0.30%