Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year	$ 37,946	$ 37,477	$ 37,391
Reinsurance recoverable on unpaid losses	11,399	11,602	12,149
Balance at beginning of year	26,547	25,875	25,242
Acquistion of subsidiaries	86	14	92
Total	26,633	25,889	25,334
Net losses and loss expenses incurred in respect of losses incurring in Current Year	9,878	10,132	10,076
Net losses and loss expenses incurred in respect of losses incurring in Prior Year	(530)	(479)	(556)
Total	9,348	9,653	9,520
Net losses and loss expenses incurred in respect of losses paid in Current Year	3,942	4,325	4,209
Net losses and loss expenses incurred in respect of losses paid in Prior Year	5,035	4,894	4,657
Total	8,977	9,219	8,866
Foreign currency revaluation and other	(173)	224	(113)
Balance at end of year	26,831	26,547	25,875
Reinsurance recoverable on unpaid losses	10,612	11,399	11,602
Gross unpaid losses and loss expenses, end of year	$ 37,443	$ 37,946	$ 37,477