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Commitments and Contingencies - Additional Information (Detail) - USD ($)
 6 Months Ended 12 Months Ended
May 13, 2014
Jun. 30, 2016
Dec. 31, 2014
Dec. 31, 2013
Loss Contingencies [Line Items]        
Civil monetary penalties $ 3,300,000      
Establishment of a restitution reserve   $ 30,000,000    
Separation and distribution agreement date   Apr. 28, 2014    
Estimated amount of credits for similar assessed late fees   $ 42,000,000    
Total reserves established during year     $ 177,000,000 $ 177,000,000
Indemnification Matters [Member] | SLM BankCo [Member]        
Loss Contingencies [Line Items]        
Reserve for estimated amounts and costs incurred   $ 0    
DOJ Order [Member]        
Loss Contingencies [Line Items]        
Civil monetary penalties     55,000  
Settlement fund under DOJ     $ 60,000,000