Unpaid losses and loss adjustment expenses - Narrative (Detail) (Property, Liability and Casualty Unpaid Losses and Loss Adjustment Expense Reserves [Member], USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Property, Liability and Casualty Unpaid Losses and Loss Adjustment Expense Reserves [Member]
Unpaid Losses and Loss Adjustment Expenses [Line Items]
Incurred losses adjustment	$ 2,544,000,000	$ 2,626,000,000	$ 1,507,000,000
Amortization charges for prior accident years' losses	249,000,000	261,000,000	504,000,000
Net discounted workers' compensation liabilities	2,250,000,000	2,315,000,000
Net discount on workers' compensation liabilities	2,130,000,000	2,269,000,000
Accretion of discount related to prior accident years' incurred losses	93,000,000	95,000,000	98,000,000
Liabilities for environmental, asbestos and latent injury claims, net of reinsurance	13,900,000,000	12,400,000,000
Liabilities for environmental, asbestos and latent injury claims, related to retroactive reinsurance contracts	$ 12,300,000,000	$ 10,700,000,000