Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (126,249)	$ (112,507)	$ (89,805)	$ (83,842)
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	4,027,332	3,427,964
Loans in forbearance	241,462	241,207
Loans in repayment and percentage of each status:
Loans current	7,678,446	6,773,095
Total Private Education Loans in repayment	7,843,076	6,927,266
Total Private Education loans, gross	12,111,870	10,596,437
Private Education Loans deferred origination costs	31,772	27,884
Total Private Education Loans	12,143,642	10,624,321
Private Education Loans allowance for losses	(122,620)	(108,816)	$ (85,236)	$ (78,574)
Private Education Loans, net	$ 12,021,022	$ 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)	64.80%	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)	3.00%	3.40%
Loans delinquent 31-60 days \| Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 78,242	$ 91,129
Loans delinquent, in percentage	1.00%	1.30%
Loans delinquent 61-90 days \| Private Education Loans
Loans in repayment and percentage of each status:
Loans delinquent	$ 56,906	$ 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	$ 29,482	$ 20,994
Loans delinquent, in percentage	0.40%	0.30%

Delinquencies as a percentage of loans in repayment.

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Loan status forbearance recorded investment.

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Loan status in school grace and deferment recorded investment.

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Total loans.

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Total loans, gross

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Loans in forbearance as a percentage of loans in repayment and forbearance.

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Total loans in repayment.

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Loans in repayment current in percentage.

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Total loans in repayment, in percentage.

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Net unamortized premium or discount on loans receivable.

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Percentage of loans in repayment.

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Percent of financing receivables past due.

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Financing receivables that are current.

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Recorded Investment in financing receivables that are past due at the balance sheet date.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

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