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Significant Accounting Policies (Details) - USD ($) $ in Thousands |  |  | 3 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 01, 2018 | Jan. 01, 2018 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2020 | Jan. 01, 2019 | Jan. 01, 2017 | Dec. 31, 2016 | Apr. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Private Education Loan loss confirmation period |  |  |  |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |
FFELP loan loss confirmation period |  |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |  |
Percentage of Private Education Loans related to borrowers inschool, grace or deferment |  |  | 25.00% |  |  |  | 26.00% |  |  |  |  | 25.00% | 26.00% |  |  |  |  |  |
Interest rate reduction and forbearance usage (greater than) |  |  |  |  |  |  |  |  |  |  |  | 3 months |  |  |  |  |  |  |
Interest rate offered to borrowers facing financial difficulty |  |  | 4.00% |  |  |  |  |  |  |  |  | 4.00% |  |  |  |  |  |  |
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) |  |  | 50.00% |  |  |  | 57.00% |  |  |  |  | 50.00% | 57.00% |  |  |  |  |  |
Percentage reimbursement on all qualifying default claims period two (as a percentage) |  |  | 97.00% |  |  |  |  |  |  |  |  | 97.00% |  |  |  |  |  |  |
Percentage reimbursement on all qualifying default claims period one (as a percentage) |  |  | 98.00% |  |  |  |  |  |  |  |  | 98.00% |  |  |  |  |  |  |
Percentage reimbursement on all qualifying default claims period three (as a percentage) |  |  | 100.00% |  |  |  |  |  |  |  |  | 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) |  |  |  |  |  |  |  |  |  |  |  | 6.92% | 6.83% |  |  |  |  |  |
Variable rate |  |  | 1.05 |  |  |  |  |  |  |  |  | 1.05 |  |  |  |  |  |  |
Gains on sales of loans, net |  |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 2,060 | $ 0 |  |  |  |  |
Asset balance related to securitization trust (as a percentage) |  |  |  |  |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |  |
Notional derivative contracts |  |  | $ 9,900,000 |  |  |  |  |  |  |  |  | $ 9,900,000 |  |  |  |  |  |  |
Basis increase to our hedged deposit balances |  |  | 52,000 |  |  |  | $ 27,000 |  |  |  |  | 52,000 | 27,000 |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 165 |  |
Gains (losses) on derivatives and hedging activities, net |  |  |  |  |  |  |  |  |  |  |  | 17,825 | (87) | (8,266) |  |  |  |  |
Income tax expense |  |  | 34,666 | $ 40,701 | $ 33,801 | $ 56,296 | 44,449 | $ (53,667) | $ 40,074 | $ 40,997 |  | 165,464 | 71,853 | 202,531 |  |  |  |  |
Discrete tax expense |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,000 |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2018-02 |  | $ 600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income taxes receivable, net |  |  | 88,844 |  |  |  | 41,570 |  |  |  |  | 88,844 | 41,570 |  |  |  |  |  |
Increase in allowance for loan losses |  |  | 441,912 |  |  |  | 341,121 |  |  |  |  | 441,912 | 341,121 | $ 251,475 |  |  |  | $ 184,701 |
Increase in exposure for unfunded commitments |  |  | 254,731 |  |  |  | 224,951 |  |  |  |  | 254,731 | 224,951 |  |  |  |  |  |
Increase in deferred tax asset |  |  | 113,900 |  |  |  | 95,804 |  |  |  |  | 113,900 | 95,804 |  |  |  |  |  |
Navient |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred tax asset discount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 27,000
Deferred tax liability |  |  | 15,000 |  |  |  |  |  |  |  |  | 15,000 |  |  |  |  |  |  | 310,000
Deferred taxes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 283,000
Income taxes receivable, net |  |  |  |  |  |  | $ 15,000 |  |  |  |  |  | 15,000 |  |  |  |  |  | 291,000
Deferred tax assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 310,000
Deferred tax asset discount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 19,000
Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (264) |  |
Reclassification resulting from the adoption of ASU No. 2018-02 |  |  |  |  |  |  |  |  |  |  |  |  | (592) |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2018-02 |  |  |  |  |  |  |  |  |  |  |  |  | $ 592 |  |  |  |  |  |
Accounting Standards Update 2017-12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 |  | 861 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2017-12 | Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 |  | 591 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2017-12 | Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 |  | $ 270 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2017-12 | Fair Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis increase to our hedged deposit balances | $ 2,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2017-12 | Fair Value | Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 | 800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2017-12 | Fair Value | Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (losses) on derivatives and hedging activities, net | 3,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2017-12 | Cash Flow | Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 | 200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Standards Update 2017-12 | Cash Flow | Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 | $ 300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustments for New Accounting Pronouncement | Accounting Standards Update 2016-09 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification resulting from the adoption of ASU No. 2017-12 |  |  |  |  |  |  |  |  |  |  | $ 200 |  |  |  |  |  |  |  |
Income tax expense |  |  |  |  |  |  |  |  |  |  | $ 8,500 |  |  |  |  |  |  |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating lease, right-of-use asset |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 34,000 |  |  |
Finance lease, liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 34,000 |  |  |
Accounting Standards Update 2016-13 | Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in allowance for loan losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |  |  |
Increase in deferred tax asset |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 300,000 |  |  |  |
Cumulative effect adjustment estimated reduction to retained earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 950,000 |  |  |  |
Accounting Standards Update 2016-13 | Forecast | Unfunded Loan Commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in exposure for unfunded commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 115,000 |  |  |  |
CME |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional derivative contracts |  |  | 9,400,000 |  |  |  |  |  |  |  |  | $ 9,400,000 |  |  |  |  |  |  |
Percent of total notional derivative contracts |  |  |  |  |  |  |  |  |  |  |  | 95.00% |  |  |  |  |  |  |
Amount of variation margin included as settlement |  |  |  |  |  |  |  |  |  |  |  | $ (107,000) |  |  |  |  |  |  |
LCH |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional derivative contracts |  |  | 500,000 |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |
Percent of total notional derivative contracts |  |  |  |  |  |  |  |  |  |  |  | 5.00% |  |  |  |  |  |  |
Amount of variation margin included as settlement |  |  |  |  |  |  |  |  |  |  |  | $ 8,000 |  |  |  |  |  |  |
Discontinued Operations, Disposed of by Sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Split Loans portfolio sales price |  |  |  |  |  |  |  |  | 43,000 |  |  |  |  |  |  |  |  |  |
Gains on sales of loans, net |  |  |  |  |  |  |  |  | $ 2,000 |  |  |  |  |  |  |  |  |  |
Split Loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Private education loans |  |  | $ 43,000 |  |  |  |  |  |  |  |  | 43,000 |  |  |  |  |  |  |
Net gain |  |  |  |  |  |  |  |  |  |  |  | $ 2,000 |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan delinquent period |  |  |  |  |  |  |  |  |  |  |  | 120 days |  |  |  |  |  |  |
Interest rate offered to borrowers facing financial difficulty |  |  |  | 4.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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