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Summary of Significant Accounting Policies Newly Adopted Accounting Standards (Detail) - USD ($)
$ in Millions
Jun. 30, 2020
Mar. 31, 2020
Jan. 01, 2020
Dec. 31, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]              
Allowance for credit loss $ 16,832     $ 7,208      
Retained earnings 35,361     40,340      
Accounting Standards Update 2016-13              
New Accounting Pronouncements or Change in Accounting Principle [Line Items]              
Deferred tax assets     $ 694        
Retained earnings     2,200        
Cumulative effects from adoption of the CECL standard              
New Accounting Pronouncements or Change in Accounting Principle [Line Items]              
Retained earnings     11        
Allowance for credit losses              
New Accounting Pronouncements or Change in Accounting Principle [Line Items]              
Allowance for credit loss $ 16,832 $ 14,073   7,208 $ 7,133 $ 7,313 $ 7,220
Finance charge and fee reserve reclassification     462        
Allowance for credit losses | Accounting Standards Update 2016-13              
New Accounting Pronouncements or Change in Accounting Principle [Line Items]              
Allowance for credit loss     $ 2,900        
Allowance for credit losses | Cumulative effects from adoption of the CECL standard              
New Accounting Pronouncements or Change in Accounting Principle [Line Items]              
Allowance for credit loss       $ 2,845