Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year	$ 37,477	$ 37,391	$ 37,783
Reinsurance recoverable on unpaid losses	11,602	12,149	12,745
Balance at beginning of year	25,875	25,242	25,038
Acquistion of subsidiaries	14	92	145
Total	25,889	25,334	25,183
Net losses and loss expenses incurred in respect of losses incurring in Current Year	10,132	10,076	8,082
Net losses and loss expenses incurred in respect of losses incurring in Prior Year	(479)	(556)	(503)
Total	9,653	9,520	7,579
Net losses and loss expenses incurred in respect of losses paid in Current Year	4,325	4,209	2,689
Net losses and loss expenses incurred in respect of losses paid in Prior Year	4,894	4,657	4,724
Total	9,219	8,866	7,413
Foreign currency revaluation and other	224	(113)	(107)
Balance at end of year	26,547	25,875	25,242
Reinsurance recoverable on unpaid losses	11,399	11,602	12,149
Gross unpaid losses and loss expenses, end of year	$ 37,946	$ 37,477	$ 37,391