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Significant Accounting Policies - Additional Information (Detail) | 3 Months Ended | 4 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Mar. 31, 2017 USD ($) | Apr. 29, 2014 | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Apr. 30, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Private Education Loan loss confirmation period (in years) |  |  | 1 year |  |  |
FFELP loan loss confirmation period (in years) |  |  | 2 years |  |  |
Loan delinquent period |  | 90 days |  |  |  |
Minimum days past due for spin off loan purchase |  |  | 90 days |  |  |
Period of hardship forbearance |  |  | 6 months |  |  |
Private education loans |  |  | $ 17,244,830,000 | $ 14,113,409,000 |  |
Personal loans held at end of year |  |  | $ 393,700,000 | $ 12,800,000.0 |  |
Interest rate reduction and forbearance usage (greater than) |  |  | 3 months |  |  |
Period of forbearance period to be classified as TDR |  |  | 3 months |  |  |
Period of forbearance |  |  | 24 months |  |  |
Period after grace period for forbearance allowance for loans |  |  | 9 months |  |  |
Forbearance period after grace period for loans |  |  | 6 months |  |  |
Percentage of loans granted forbearance (as a percentage) |  |  | 66.00% | 69.00% |  |
Percentage of Private Education Loans related to borrowers inschool, grace or deferment (as a percentage) |  |  | 27.00% | 29.00% |  |
Period after last payment contractually due private education loan considered to be delinquent |  |  | 31 days |  |  |
Percentage reimbursement on all qualifying default claims period two (as a percentage) |  |  | 97.00% |  |  |
Percentage reimbursement on all qualifying default claims period one (as a percentage) |  |  | 98.00% |  |  |
Percentage reimbursement on all qualifying default claims period three (as a percentage) |  |  | 100.00% |  |  |
Period of loss emergence to estimate credit losses incurred |  |  | 2 years |  |  |
Period of notification for withdrawal of deposits (less than or equal to) |  |  | 7 days |  |  |
Constant prepayment rate for Private Education Loans (in percentage) |  |  | 5.94% | 5.00% |  |
Variable rate |  |  | 1.05 |  |  |
Gains on sales of loans, net |  |  | $ 0 | $ 230,000 | $ 135,358,000 |
Asset balance related to securitization trust (as a percentage) |  |  | 10.00% |  |  |
Total amount of notional derivative contracts |  |  | $ 5,500,000,000 |  |  |
Excess tax benefit, amount | $ 200,000 |  | 8,500,000 |  |  |
Deferred taxes |  |  |  | 9,180,000 |  | $ 283,000,000
Tax indemnification receivable |  |  | 168,011,000 | 259,532,000 |  |
Deferred tax asset discount |  |  | 108,000,000 |  |  |
Remaining amount of indemnification |  |  | $ 35,000,000 |  |  |
Maximum |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Loan delinquent period |  |  | 120 days |  |  |
Split Loans |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Private education loans |  |  | $ 48,000,000 |  |  |
Navient |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Deferred taxes |  |  | 37,000,000 |  |  | 283,000,000
Tax indemnification receivable |  |  |  | 60,000,000 |  | 291,000,000
Deferred tax asset discount |  |  | 25,000,000 | 25,000,000 |  | 27,000,000
Deferred tax liability |  |  | 62,000,000 |  |  | 310,000,000
Deferred tax assets |  |  |  |  |  | 310,000,000
Deferred tax asset discount |  |  |  |  |  | $ 19,000,000
Remaining amount of indemnification |  |  |  | 35,000,000 |  |
Remaining balance of liability for uncertain tax positions |  |  | 108 |  |  |
Additional Tax Positions |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Increase in tax receivable and income tax payable |  |  |  | 120,000,000 |  |
Increase in other income and income tax expense |  |  |  | $ 9,000,000 |  |
CME |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total amount of notional derivative contracts |  |  | $ 4,800,000,000 |  |  |
Percent of total notional derivative contracts |  |  | 87.60% |  |  |
LCH |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Total amount of notional derivative contracts |  |  | $ 700,000,000 |  |  |
Percent of total notional derivative contracts |  |  | 12.40% |  |  |

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