Allowance for Loan Losses - Age Analysis of Past Due Loan Delinquencies (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
TDR loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (184,643)	$ (112,507)	$ (83,842)	$ (68,081)
Student Loan \| Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	4,189,955	3,427,964	3,027,143
Loans in forbearance	351,962	241,207	135,018
TDR loans in repayment and percentage of each status:
Loans current	9,509,394	6,773,095	5,045,600
Total TDR loans in repayment	$ 9,709,758	$ 6,927,266	$ 5,149,215
Loans current, in percentage	97.90%	97.80%	98.00%
Loans in repayment, in percentage	100.00%	100.00%	100.00%
Total TDR loans, gross	$ 14,251,675	$ 10,596,437	$ 8,311,376
Private Education Loans deferred origination costs	44,206	27,884	13,845
Total Private Education Loans	14,295,881	10,624,321	8,325,221
Private Education Loans allowance for losses	(182,472)	(108,816)	(78,574)
Percentage of Private Education Loans in repayment	$ 14,113,409	$ 10,515,505	$ 8,246,647
Percentage of Private Education Loans in repayment	68.10%	65.40%	62.00%
Delinquencies as a percentage of Private Education Loans in repayment	2.10%	2.20%	2.00%
Loans in forbearance as a percentage of Private Education Loans in repayment and forbearance	3.50%	3.40%	2.60%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 124,773	$ 91,129	$ 63,873
Loans delinquent (in percentage)	1.30%	1.30%	1.20%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 51,423	$ 42,048	$ 29,041
Loans delinquent (in percentage)	0.50%	0.60%	0.60%
Student Loan \| Consumer Portfolio Segment \| Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 24,168	$ 20,994	$ 10,701
Loans delinquent (in percentage)	0.30%	0.30%	0.20%
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	$ 24,185	$ 6,869
Loans in forbearance	71,851	43,756
TDR loans in repayment and percentage of each status:
Loans current	462,187	185,936
Total TDR loans in repayment	$ 516,570	$ 215,206
Loans current, in percentage	89.50%	86.40%
Loans in repayment, in percentage	100.00%	100.00%
Total TDR loans, gross	$ 612,606	$ 265,831
Troubled Debt Restructured Loans \| Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 28,452	$ 14,948
Loans delinquent (in percentage)	5.50%	6.90%
Troubled Debt Restructured Loans \| Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 17,326	$ 9,239
Loans delinquent (in percentage)	3.40%	4.30%
Troubled Debt Restructured Loans \| Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 8,605	$ 5,083
Loans delinquent (in percentage)	1.60%	2.40%

Delinquencies as a percentage of loans in repayment.

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Total loans.

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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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