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, 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 and percentage of each status.

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Loans in repayment current in percentage.

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Total loans in repayment, in percentage.

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Percentage of loans in repayment.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage of financing receivable balance that is 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, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

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

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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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Allowance for Loan Losses - Loan Status and Aging of Past Due TDR Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
TDR loans in repayment and percentage of each status:
Loans held for investment, net	$ 20,166,604	$ 18,567,641
Allowance for loan losses	(272,123)	(251,475)	$ (187,086)	$ (184,701)
Student Loan \| Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	5,369,984	4,757,732
TDR loans in forbearance	465,286	468,402
TDR loans in repayment and percentage of each status:
Loans current	12,635,627	11,911,128
Total TDR loans in repayment	$ 12,958,742	$ 12,206,033
Loans current (as a percentage)	97.50%	97.60%
Total TDR loans in repayment	100.00%	100.00%
Loans Gross	$ 18,794,012	$ 17,432,167
Private Education Loans deferred origination costs	58,814	56,378
Loans and Leases Receivable, before Fees, Gross	18,852,826	17,488,545
Loans held for investment, net	18,600,723	17,244,830
Allowance for loan losses	$ (252,103)	$ (243,715)
Percentage of Private Education Loans in repayment	69.00%	70.00%
Delinquencies as a percentage of Private Education Loans in repayment	2.50%	2.40%
Loans in forbearance as a percentage of Private Education Loans in repayment and forbearance	3.50%	3.70%
Student Loan \| Consumer Portfolio Segment \| Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	$ 58,939	$ 51,745
TDR loans in forbearance	65,036	69,652
TDR loans in repayment and percentage of each status:
Loans current	823,813	774,222
Total TDR loans in repayment	$ 919,128	$ 868,954
Loans current (as a percentage)	89.70%	89.10%
Total TDR loans in repayment	100.00%	100.00%
Total Private Education Loans	$ 1,043,103	$ 990,351
Student Loan \| Consumer Portfolio Segment \| Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 179,989	$ 179,002
Loans delinquent (as a percentage)	1.40%	1.50%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent 31-60 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 47,127	$ 48,377
Loans delinquent (as a percentage)	5.10%	5.60%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 95,974	$ 78,292
Loans delinquent (as a percentage)	0.70%	0.60%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent 61-90 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 31,463	$ 28,778
Loans delinquent (as a percentage)	3.40%	3.30%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent, greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 47,152	$ 37,611
Loans delinquent (as a percentage)	0.40%	0.30%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent, greater than 90 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 16,725	$ 17,577
Loans delinquent (as a percentage)	1.80%	2.00%