Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (272,123)	$ (251,475)	$ (187,086)	$ (184,701)
Loans held for investment, net	20,166,604	18,567,641
Private Education Loans
Loans in repayment and percentage of each status:
Total Private Education Loans, gross	18,794,012	17,432,167
Private Education Loans deferred origination costs	58,814	56,378
Private Education Loans allowance for losses	(252,103)	(243,715)
Loans held for investment, net	$ 18,600,723	$ 17,244,830

Loans in repayment and percentage of each status.

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- Definition

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.

+ References

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Amount, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

+ References

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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. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

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Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

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