Pension plans - Fair value of plan assets (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 9,150	$ 8,246	$ 5,926
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	830	474
US Treasury and Government [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	915	895
Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,872	2,020
Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,180	1,015
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,618	3,069
Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	735	773
Quoted Prices (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,297	4,899
Quoted Prices (Level 1) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	797	423
Quoted Prices (Level 1) [Member] | US Treasury and Government [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	534	609
Quoted Prices (Level 1) [Member] | Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	402	597
Quoted Prices (Level 1) [Member] | Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	95	147
Quoted Prices (Level 1) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,432	3,069
Quoted Prices (Level 1) [Member] | Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37	54
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,463	2,976
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33	51
Significant Other Observable Inputs (Level 2) [Member] | US Treasury and Government [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	380	285
Significant Other Observable Inputs (Level 2) [Member] | Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,465	1,423
Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,085	868
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	186
Significant Other Observable Inputs (Level 2) [Member] | Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	314	349
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	390	371
Significant Unobservable Inputs (Level 3) [Member] | US Treasury and Government [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Significant Unobservable Inputs (Level 3) [Member] | Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5
Significant Unobservable Inputs (Level 3) [Member] | Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 384	$ 370