Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Loans in repayment and percentage of each status:
Allowance for loan losses	$ (104,203)	$ (83,842)	$ (91,866)	$ (65,715)	$ (60,527)	$ (68,081)
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	3,971,392	3,027,143
Loans in forbearance	211,641	135,018
Loans in repayment and percentage of each status:
Loans current	6,529,855	5,045,600
Total loans in repayment	6,657,228	5,149,215
Total loans, gross	10,840,261	8,311,376
Deferred origination costs	26,283	13,845
Total loans	10,866,544	8,325,221
Allowance for loan losses	(100,033)	(78,574)	$ (87,310)	$ (59,973)	$ (54,315)	$ (61,763)
Total loans, net	$ 10,766,511	$ 8,246,647
Loans current, in percentage	98.10%	98.00%
Total loans in repayment, in percentage	100.00%	100.00%
Percentage of loans in repayment	61.40%	62.00%
Delinquencies as a percentage of loans in repayment	1.90%	2.00%
Loans in forbearance as a percentage of loans in repayment and forbearance	3.10%	2.60%
Loans delinquent 31-60 days | Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 79,794	$ 63,873
Loans delinquent, in percentage	1.20%	1.20%
Loans delinquent 61-90 days | Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 34,743	$ 29,041
Loans delinquent, in percentage	0.50%	0.60%
Loans delinquent greater than 90 days | Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 12,836	$ 10,701
Loans delinquent, in percentage	0.20%	0.20%