Pension plans - Fair value of plan assets (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 17,938	$ 16,496	$ 14,959
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	383	412
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11,383	11,105
Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,173	2,328
Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,999	2,651
Quoted Prices (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12,778	11,905
Quoted Prices (Level 1) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	243	309
Quoted Prices (Level 1) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,123	9,860
Quoted Prices (Level 1) [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,214	1,593
Quoted Prices (Level 1) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	198	143
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,315	2,001
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	140	103
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	851	836
Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	926	704
Significant Other Observable Inputs (Level 2) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	398	358
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	498	480
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	409	409
Significant Unobservable Inputs (Level 3) [Member] | Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33	31
Significant Unobservable Inputs (Level 3) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	56	40
Investment Funds and Partnerships at Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,347	2,110
Investment Funds and Partnerships at Net Asset Value [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,347	$ 2,110