Investments in equity securities (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Summary of Investment Holdings [Line Items]    
Cost Basis $ 102,867 $ 74,676
Net Unrealized Gains 69,890 95,864
Fair Value 172,757 [1] 170,540 [2]
Banks, insurance and finance [Member]    
Summary of Investment Holdings [Line Items]    
Cost Basis 44,332 25,783
Net Unrealized Gains 38,260 55,026
Fair Value 82,592 80,809
Consumer products [Member]    
Summary of Investment Holdings [Line Items]    
Cost Basis 38,783 25,177
Net Unrealized Gains 22,838 25,698
Fair Value 61,621 50,875
Commercial, industrial and other [Member]    
Summary of Investment Holdings [Line Items]    
Cost Basis 19,752 23,716
Net Unrealized Gains 8,792 15,140
Fair Value $ 28,544 $ 38,856
[1] Approximately 68% of the aggregate fair value was concentrated in five companies (American Express Company - $14.5 billion; Apple Inc. - $40.3 billion; Bank of America Corporation - $22.6 billion; The Coca-Cola Company - $18.9 billion and Wells Fargo & Company - $20.7 billion).
[2] Approximately 65% of the aggregate fair value was concentrated in five companies (American Express Company - $15.1 billion; Apple Inc. - $28.2 billion; Bank of America Corporation - $20.7 billion; The Coca-Cola Company - $18.4 billion and Wells Fargo & Company - $29.3 billion).