Pension plans - Additional tabular disclosures (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Assumptions applicable to pension benefit obligations and pension expense:
Discount rate	3.80%	4.60%
Expected long-term rate of return on plan assets	6.70%	6.70%
Rate of compensation increase	3.40%	3.50%
Discount rate applicable to pension expense	4.60%	4.10%
Amounts recognized in the Consolidated Balance Sheets:
Net funded status	$ 2,521	$ 621
Amounts recognized in other comprehensive income:
Accumulated other comprehensive income (loss), beginning of year	86	(2,516)
Amount included in net periodic pension expense	55	167
Gains (losses) current period and other	(1,755)	2,435
Accumulated other comprehensive income (loss), end of year	(1,614)	86
Accounts Payable, Accruals and Other Liabilities [Member]
Amounts recognized in the Consolidated Balance Sheets:
Net funded status	2,550	1,287
Losses and Loss Adjustment Expenses [Member]
Amounts recognized in the Consolidated Balance Sheets:
Net funded status	332	309
Other Assets [Member]
Amounts recognized in the Consolidated Balance Sheets:
Excess of plan assets over plan obligations	$ (361)	$ (975)