Allowance for Loan Losses - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Jul. 01, 2006
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Interest rate offered to borrowers facing financial difficulty	4.00%
Interest rate offered to borrowers facing financial difficulty, period	2 years
Percentage of loans in full and interest repayment status that are subject to interest rate reductions	7.20%		6.40%
Percentage of loans granted forbearance (as a percentage)	50.00%		57.00%
Criteria for loans to be considered as nonperforming	90 days
Tier 1 of government guarantee (at least)				97.00%
TDR payment default period	60 days
Period of loans past due that have accrued interest	90 days
FFELP Loans
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Tier 1 of government guarantee (at least)	97.00%
Maximum
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Interest rate offered to borrowers facing financial difficulty		4.00%
Minimum
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables Table [Line Items]
Interest rate offered to borrowers facing financial difficulty		2.00%