Pension plans - Fair value of plan assets (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 10,436	$ 9,150	$ 8,246
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	900	830
Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	899	915
Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,069	1,872
Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	689	1,180
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,444	3,618
Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	435	735
Quoted Prices (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,596	5,297
Quoted Prices (Level 1) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	345	797
Quoted Prices (Level 1) [Member] | Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	529	534
Quoted Prices (Level 1) [Member] | Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	413	402
Quoted Prices (Level 1) [Member] | Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	86	95
Quoted Prices (Level 1) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,211	3,432
Quoted Prices (Level 1) [Member] | Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	37
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,508	3,463
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	555	33
Significant Other Observable Inputs (Level 2) [Member] | Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	370	380
Significant Other Observable Inputs (Level 2) [Member] | Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,650	1,465
Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	603	1,085
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	233	186
Significant Other Observable Inputs (Level 2) [Member] | Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	97	314
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	332	390
Significant Unobservable Inputs (Level 3) [Member] | Government Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1
Significant Unobservable Inputs (Level 3) [Member] | Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	5
Significant Unobservable Inputs (Level 3) [Member] | Other Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 326	$ 384