Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2015	Dec. 31, 2016	Mar. 31, 2016
Loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (187,086)	$ (112,507)	$ (184,701)	$ (126,249)
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	4,778,295		4,189,955
Loans in forbearance	349,777		351,962
Loans in repayment and percentage of each status:
Loans current	$ 10,327,843		9,509,394
Loans current (as a percentage)	98.10%	97.90%
Total TDR loans in repayment	$ 10,526,782		9,709,758
Total TDR loans in repayment	100.00%	100.00%
Total TDR loans, gross	$ 15,654,854		14,251,675
Private Education Loans deferred origination costs	46,692		44,206
Total Private Education Loans	15,701,546		14,295,881
Private Education Loans allowance for losses	(185,103)		(182,472)
Private Education Loans, net	$ 15,516,443		$ 14,113,409
Percentage of Private Education Loans in repayment	67.20%		68.10%
Delinquencies as a percentage of Private Education Loans in repayment	1.90%		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	$ 112,167		$ 124,773
Loans delinquent (as a percentage)	1.10%	1.30%
Student Loan | Consumer Portfolio Segment | Loans delinquent 61-90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 54,128		51,423
Loans delinquent (as a percentage)	0.50%	0.50%
Student Loan | Consumer Portfolio Segment | Loans delinquent, greater than 90 days
Loans in repayment and percentage of each status:
Loans delinquent	$ 32,644		$ 24,168
Loans delinquent (as a percentage)	0.30%	0.30%