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Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
SecurityLoan
Sep. 30, 2015
USD ($)
SecurityLoan
Jun. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
SecurityLoan
Sep. 30, 2015
USD ($)
SecurityLoan
Allowance for Loan Losses          
Beginning balance $ 1,487 $ 1,643   $ 1,564 $ 2,033
Total provision 106 123   327 446
Net adjustment resulting from the change in the charge-off rate         (330)
Net charge-offs remaining         (549)
Total net charge-offs (130) (161)   (434) 879
Reclassification of interest reserve 2 3   8 8
Ending balance 1,465 1,608 $ 1,643 1,465 1,608
Allowance:          
Ending balance: individually evaluated for impairment 1,164 1,263   1,164 1,263
Ending balance: collectively evaluated for impairment 301 345   301 345
Loans:          
Ending balance: individually evaluated for impairment 11,214 10,911   11,214 10,911
Ending balance: collectively evaluated for impairment 104,072 115,982   104,072 115,982
FFELP Loans [Member]          
Allowance for Loan Losses          
Beginning balance 62 89   78 93
Total provision 13 7   30 19
Net charge-offs remaining         (28)
Total net charge-offs (17) (12)   (50) 28
Ending balance 58 84 89 58 84
Allowance:          
Ending balance: collectively evaluated for impairment 58 84   58 84
Loans:          
Ending balance: collectively evaluated for impairment $ 89,291 $ 97,425   $ 89,291 $ 97,425
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in the charge-off rate (annualized)         0.05%
Net charge-offs as a percentage of average loans in repayment (annualized) 0.09% 0.06%   0.09%  
Allowance coverage of net charge-offs (annualized) | SecurityLoan 0.8 1.7   0.9  
Allowance coverage of net charge-offs, excluding the net adjustment resulting from the change in the charge-off rate (annualized) | SecurityLoan         2.2
Allowance as a percentage of the ending total loan balance 0.07% 0.09%   0.07% 0.09%
Allowance as a percentage of the ending loans in repayment 0.08% 0.11%   0.08% 0.11%
Ending total loans $ 89,291 $ 97,425   $ 89,291 $ 97,425
Average loans in repayment 72,921 75,460   73,193 76,412
Ending loans in repayment 72,269 75,294   72,269 75,294
Private Education Loans [Member]          
Allowance for Loan Losses          
Beginning balance 1,410 1,533   1,471 1,916
Total provision 92 117   296 428
Net adjustment resulting from the change in the charge-off rate     (330)   (330)
Net charge-offs remaining         (517)
Total net charge-offs (112) (148)   (383) 847
Reclassification of interest reserve 2 3   8 8
Ending balance 1,392 1,505 1,533 1,392 1,505
Allowance:          
Ending balance: individually evaluated for impairment 1,153 1,247   1,153 1,247
Ending balance: collectively evaluated for impairment 239 258   239 258
Loans:          
Ending balance: individually evaluated for impairment 11,182 10,870   11,182 10,870
Ending balance: collectively evaluated for impairment $ 14,682 $ 18,507   $ 14,682 $ 18,507
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in the charge-off rate (annualized)         2.65%
Net charge-offs as a percentage of average loans in repayment (annualized) 1.94% 2.31%   2.17%  
Net adjustment resulting from the change in the charge-off rate as a percentage of average loans in repayment (annualized)         1.69%
Allowance coverage of net charge-offs (annualized) | SecurityLoan 3.1 2.6   2.7  
Allowance coverage of net charge-offs, excluding the net adjustment resulting from the change in the charge-off rate (annualized) | SecurityLoan         2.2
Allowance as a percentage of the ending total loan balance 5.38% 5.12%   5.38% 5.12%
Allowance as a percentage of the ending loans in repayment 6.17% 5.99%   6.17% 5.99%
Ending total loans $ 25,864 $ 29,377   $ 25,864 $ 29,377
Average loans in repayment 22,959 25,546   23,564 26,100
Ending loans in repayment 22,556 25,104   22,556 25,104
Other Loans [Member]          
Allowance for Loan Losses          
Beginning balance 15 21   15 24
Total provision 1 (1)   1 (1)
Net charge-offs remaining         (4)
Total net charge-offs (1) (1)   (1) 4
Ending balance 15 19 $ 21 15 19
Allowance:          
Ending balance: individually evaluated for impairment 11 16   11 16
Ending balance: collectively evaluated for impairment 4 3   4 3
Loans:          
Ending balance: individually evaluated for impairment 32 41   32 41
Ending balance: collectively evaluated for impairment $ 99 $ 50   $ 99 $ 50
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in the charge-off rate (annualized)         5.72%
Net charge-offs as a percentage of average loans in repayment (annualized) 1.87% 5.10%   1.76%  
Allowance coverage of net charge-offs (annualized) | SecurityLoan 7.9 3.9   9.5  
Allowance coverage of net charge-offs, excluding the net adjustment resulting from the change in the charge-off rate (annualized) | SecurityLoan         3.3
Allowance as a percentage of the ending total loan balance 11.40% 20.31%   11.40% 20.31%
Allowance as a percentage of the ending loans in repayment 11.40% 20.31%   11.40% 20.31%
Ending total loans $ 131 $ 91   $ 131 $ 91
Average loans in repayment 102 93   89 99
Ending loans in repayment $ 131 $ 91   $ 131 $ 91