Pension plans - Fair value of plan assets (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 16,496	$ 14,959	$ 17,014
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	412	1,328
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11,105	7,671
Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,537	1,727
Other Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	791	836
Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,651	3,397
Quoted Prices (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11,905	10,692
Quoted Prices (Level 1) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	309	1,197
Quoted Prices (Level 1) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,860	7,499
Quoted Prices (Level 1) [Member] | Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,433	1,654
Quoted Prices (Level 1) [Member] | Other Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160	172
Quoted Prices (Level 1) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	143	170
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,001	1,899
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103	131
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	836	22
Significant Other Observable Inputs (Level 2) [Member] | Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	104	73
Significant Other Observable Inputs (Level 2) [Member] | Other Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	600	631
Significant Other Observable Inputs (Level 2) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	358	1,042
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	480	456
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	409	150
Significant Unobservable Inputs (Level 3) [Member] | Other Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	33
Significant Unobservable Inputs (Level 3) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40	273
Investment Funds and Partnerships at Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,110	1,912
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	$ 2,110	$ 1,912