Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (229,919)	$ (207,448)	$ (184,701)	$ (164,839)	$ (144,925)	$ (112,507)
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	5,352,671		4,189,955
Loans in forbearance	373,655		351,962
Loans in repayment and percentage of each status:
Loans current	$ 11,115,697		$ 9,509,394
Loans current (as a percentage)	97.40%		97.90%
Total TDR loans in repayment	$ 11,406,581		$ 9,709,758
Total TDR loans in repayment	100.00%		100.00%
Total TDR loans, gross	$ 17,132,907		$ 14,251,675
Private Education Loans deferred origination costs	53,501		44,206
Total Private Education Loans	17,186,408		14,295,881
Private Education Loans allowance for losses	(227,167)		(182,472)
Private Education Loans, net	$ 16,959,241		$ 14,113,409
Percentage of Private Education Loans in repayment	66.60%		68.10%
Delinquencies as a percentage of Private Education Loans in repayment	2.60%		2.10%
Loans in forbearance as a percentage of Private Education Loans in repayment and forbearance	3.20%		3.50%
Student Loan | Consumer Portfolio Segment | Loans delinquent 31-60 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 175,897		$ 124,773
Loans delinquent (as a percentage)	1.60%		1.30%
Student Loan | Consumer Portfolio Segment | Loans delinquent 61-90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 82,095		$ 51,423
Loans delinquent (as a percentage)	0.70%		0.50%
Student Loan | Consumer Portfolio Segment | Loans delinquent, greater than 90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 32,892		$ 24,168
Loans delinquent (as a percentage)	0.30%		0.30%