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Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Mar. 31, 2021
Dec. 31, 2020
Mar. 31, 2020
Dec. 31, 2019
Allowance for Loan Losses                
Allowance at beginning of period $ 1,274.0 $ 1,394.0 $ 1,377.0 $ 1,112.0        
Provision:                
Reversal of allowance related to loan sales (5.0)   (107.0)          
Remaining provision 4.0   19.0          
Total provision (1.0) 44.0 (88.0) 139.0        
Charge-offs(2) (40.0) (60.0) (81.0) (147.0)        
Decrease in expected future recoveries on charged-off loans 20.0 22.0 45.0 39.0        
Allowance at end of period 1,253.0 1,400.0 1,253.0 1,400.0        
Plus: expected future recoveries on charged-off loans 434.0 549.0 434.0 549.0        
Allowance at end of period excluding expected future recoveries on charged-off loans 1,687.0 1,949.0 1,687.0 1,949.0        
ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment                
Allowance for Loan Losses                
Allowance at beginning of period       257.0        
ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]                
Allowance for Loan Losses                
Allowance at beginning of period       1,369.0        
FFELP Loans [Member]                
Allowance for Loan Losses                
Allowance at beginning of period 282.0 311.0 288.0 64.0        
Provision:                
Total provision   3.0   9.0        
Charge-offs(2) (5.0) (12.0) (11.0) (31.0)        
Allowance at end of period 277.0 302.0 277.0 302.0        
Allowance at end of period excluding expected future recoveries on charged-off loans $ 277.0 $ 302.0 $ 277.0 $ 302.0        
Charge-offs as a percentage of average loans in repayment (annualized) 0.04% 0.11% 0.05% 0.13%        
Allowance coverage of charge-offs (annualized) 15.5 6.3 12.5 4.8        
Allowance as a percentage of the ending total loan balance 0.50% 0.50% 0.50% 0.50%        
Allowance as a percentage of ending loans in repayment 0.60% 0.70% 0.60% 0.70%        
Ending total loans $ 55,827.0 $ 61,223.0 $ 55,827.0 $ 61,223.0        
Average loans in repayment 46,348.0 44,144.0 46,694.0 48,302.0        
Ending loans in repayment 45,854.0 42,640.0 45,854.0 42,640.0        
FFELP Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment                
Allowance for Loan Losses                
Allowance at beginning of period       260.0        
FFELP Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]                
Allowance for Loan Losses                
Allowance at beginning of period       324.0        
Private Education Loans [Member]                
Allowance for Loan Losses                
Allowance at beginning of period 992.0 1,083.0 1,089.0 1,048.0        
Provision:                
Reversal of allowance related to loan sales (5.0)   (107.0)          
Remaining provision 4.0   19.0          
Total provision (1.0) 41.0 (88.0) 130.0        
Charge-offs(2) (35.0) (48.0) (70.0) (116.0)        
Decrease in expected future recoveries on charged-off loans 20.0 22.0 45.0 39.0        
Allowance at end of period 976.0 1,098.0 976.0 1,098.0        
Plus: expected future recoveries on charged-off loans 434.0 549.0 434.0 549.0 $ 454.0 $ 479.0 $ 571.0 $ 588.0
Allowance at end of period excluding expected future recoveries on charged-off loans $ 1,410.0 $ 1,647.0 $ 1,410.0 $ 1,647.0        
Charge-offs as a percentage of average loans in repayment (annualized) 0.71% 0.97% 0.70% 1.13%        
Allowance coverage of charge-offs (annualized) 10.0 8.6 10.0 7.1        
Allowance as a percentage of the ending total loan balance 6.80% 7.30% 6.80% 7.30%        
Allowance as a percentage of ending loans in repayment 7.20% 8.20% 7.20% 8.20%        
Ending total loans $ 20,701.0 $ 22,560.0 $ 20,701.0 $ 22,560.0        
Average loans in repayment 19,667.0 19,731.0 20,272.0 20,666.0        
Ending loans in repayment $ 19,692.0 $ 20,201.0 $ 19,692.0 20,201.0        
Private Education Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment                
Allowance for Loan Losses                
Allowance at beginning of period       (3.0)        
Private Education Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]                
Allowance for Loan Losses                
Allowance at beginning of period       $ 1,045.0