Pension plans - Additional tabular disclosures (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Amounts recognized in the Consolidated Balance Sheets:
Unfunded defined benefit pension cost obligations	$ 3,637	$ 3,842
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income
Accumulated other comprehensive income (loss), beginning of year	(2,521)	(1,395)
Amount included in net periodic pension expense	130	76
Gains (losses) current period and other	(125)	(1,202)
Accumulated other comprehensive income (loss), end of year	(2,516)	(2,521)
Assumptions applicable to pension benefit obligations:
Discount rate	4.00%	4.60%
Expected long-term rate of return on plan assets	6.60%	6.90%
Rate of compensation increase	3.60%	3.70%
Discount rate applicable to pension expense	4.50%	5.30%
Accounts Payable, Accruals and Other Liabilities [Member]
Amounts recognized in the Consolidated Balance Sheets:
Unfunded defined benefit pension cost obligations	3,441	3,686
Losses and Loss Adjustment Expenses [Member]
Amounts recognized in the Consolidated Balance Sheets:
Unfunded defined benefit pension cost obligations	256	214
Other Assets [Member]
Amounts recognized in the Consolidated Balance Sheets:
Excess of plan assets over plan obligations	$ (60)	$ (58)