Pension plans - Fair value of plan assets (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 12,831	$ 13,366	$ 13,277
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	839	482
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,319	7,950
Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	786	811
Other Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	990	908
Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,897	3,215
Quoted Prices (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,019	9,352
Quoted Prices (Level 1) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	544	250
Quoted Prices (Level 1) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,305	7,739
Quoted Prices (Level 1) [Member] | Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	726	701
Quoted Prices (Level 1) [Member] | Other Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87	67
Quoted Prices (Level 1) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	357	595
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,472	3,681
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	295	232
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	14	211
Significant Other Observable Inputs (Level 2) [Member] | Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	60	110
Significant Other Observable Inputs (Level 2) [Member] | Other Fixed Maturity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	903	841
Significant Other Observable Inputs (Level 2) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,200	2,287
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	340	333
Significant Unobservable Inputs (Level 3) [Member] | Investment Funds and Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 340	$ 333