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Pension plans - Fair value of plan assets (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 13,379 | $ 12,592 | $ 18,620
Cash and Cash Equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 449 | 602 |
Equity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 8,487 | 7,673 |
Fixed Maturity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2,138 | 2,152 |
Investment Funds and Other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2,305 | 2,165 |
Quoted Prices (Level 1) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 9,797 | 9,161 |
Quoted Prices (Level 1) [Member] | Cash and Cash Equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 370 | 523 |
Quoted Prices (Level 1) [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 7,808 | 7,112 |
Quoted Prices (Level 1) [Member] | Fixed Maturity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1,277 | 1,328 |
Quoted Prices (Level 1) [Member] | Investment Funds and Other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 342 | 198 |
Significant Other Observable Inputs (Level 2) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1,745 | 1,618 |
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash Equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 79 | 79 |
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 543 | 321 |
Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 851 | 824 |
Significant Other Observable Inputs (Level 2) [Member] | Investment Funds and Other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 272 | 394 |
Significant Unobservable Inputs (Level 3) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 188 | 282 |
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 136 | 240 |
Significant Unobservable Inputs (Level 3) [Member] | Fixed Maturity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 10 |  |
Significant Unobservable Inputs (Level 3) [Member] | Investment Funds and Other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 42 | 42 |
Investments Carried at Net Asset Value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1,649 | 1,531 |
Investments Carried at Net Asset Value [Member] | Investment Funds and Other [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 1,649 | $ 1,531 |

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