Pension plans - Additional tabular disclosures (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assumptions applicable to pension benefit obligations and pension expense:
Discount rate	4.10%	3.80%	4.60%
Expected long-term rate of return on plan assets	6.50%	6.70%	6.70%
Rate of compensation increase	3.40%	3.40%	3.50%
Discount rate applicable to pension expense	3.80%	4.60%	4.10%
Amounts recognized in the Consolidated Balance Sheets:
Net funded status	$ 2,428	$ 2,521
Amounts recognized in other comprehensive income:
Accumulated other comprehensive income (loss), beginning of year	(1,617)	86
Amount included in net periodic pension expense	129	58
Gains (losses) current period and other	295	(1,761)
Accumulated other comprehensive income (loss), end of year	(1,193)	(1,617)	$ 86
Accounts Payable, Accruals and Other Liabilities [Member]
Amounts recognized in the Consolidated Balance Sheets:
Net funded status	2,484	2,550
Losses and Loss Adjustment Expenses [Member]
Amounts recognized in the Consolidated Balance Sheets:
Net funded status	400	332
Other Assets [Member]
Amounts recognized in the Consolidated Balance Sheets:
Excess of plan assets over plan obligations	$ (456)	$ (361)