Significant Accounting Policies - Troubled Debt Restructurings (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Percentage of Private Education Loans related to borrowers in school, grace or deferment	24.00%	25.00%
Interest rate offered to borrowers facing financial difficulty	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)	47.00%	50.00%

Loans and Leases Receivable, Forbearance Period after Grace Period

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Loans and Leases Receivable, Period After Grace Period for Forbearance Allowance

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Loans and Leases Receivable, Period of Forbearance

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Loans and Leases Receivable, Period of Forbearance Within Twenty-Four Month Period to be Classified as TDR

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Loans And Leases Receivable, Temporary Interest Rate Reduction, Financial Difficulty Status

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Loans and Leases Receivable, Impaired, Loans Granted Forbearance Qualified as Trouble Debt Restructuring

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Percentage Of Private Education Loans Related To Borrowers In School, Grace or Deferment

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