Pension plans - Additional tabular disclosures (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Amounts recognized in the Consolidated Balance Sheets:
Unfunded defined benefit pension cost obligations	$ 621	$ 3,637
Amounts recognized in other comprehensive income:
Accumulated other comprehensive income (loss), beginning of year	(2,516)	(2,521)
Amount included in net periodic pension expense	167	130
Gains (losses) current period and other	2,435	(125)
Accumulated other comprehensive income (loss), end of year	86	(2,516)
Assumptions applicable to pension benefit obligations and pension expense:
Discount rate	4.60%	4.00%
Expected long-term rate of return on plan assets	6.70%	6.60%
Rate of compensation increase	3.50%	3.60%
Discount rate applicable to pension expense	4.10%	4.50%
Accounts Payable, Accruals and Other Liabilities [Member]
Amounts recognized in the Consolidated Balance Sheets:
Unfunded defined benefit pension cost obligations	1,287	3,441
Losses and Loss Adjustment Expenses [Member]
Amounts recognized in the Consolidated Balance Sheets:
Unfunded defined benefit pension cost obligations	309	256
Other Assets [Member]
Amounts recognized in the Consolidated Balance Sheets:
Excess of plan assets over plan obligations	$ (975)	$ (60)