Investments in equity securities (Parenthetical) (Detail) $ in Millions |
Dec. 31, 2018
USD ($)
Company
|
Dec. 31, 2017
USD ($)
Company
|
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|---|---|---|---|---|---|---|---|---|
| Summary of Investment Holdings [Line Items] | ||||||||
| Fair value of investments | $ 172,757 | [1] | $ 170,540 | [2] | ||||
| American Express Company [Member] | ||||||||
| Summary of Investment Holdings [Line Items] | ||||||||
| Fair value of investments | 14,500 | 15,100 | ||||||
| Apple Inc. [Member] | ||||||||
| Summary of Investment Holdings [Line Items] | ||||||||
| Fair value of investments | 40,300 | 28,200 | ||||||
| Bank of America Corporation [Member] | ||||||||
| Summary of Investment Holdings [Line Items] | ||||||||
| Fair value of investments | 22,600 | 20,700 | ||||||
| The Coca-Cola Company [Member] | ||||||||
| Summary of Investment Holdings [Line Items] | ||||||||
| Fair value of investments | 18,900 | 18,400 | ||||||
| Wells Fargo & Company [Member] | ||||||||
| Summary of Investment Holdings [Line Items] | ||||||||
| Fair value of investments | $ 20,700 | $ 29,300 | ||||||
| Equity Securities [Member] | Investment Concentration [Member] | ||||||||
| Summary of Investment Holdings [Line Items] | ||||||||
| Concentration percentage | 68.00% | 65.00% | ||||||
| Number of companies in concentration percentage | Company | 5 | 5 | ||||||
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