Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year	$ 37,443		$ 37,946		$ 37,477
Reinsurance recoverable on unpaid losses	(10,612)	[1]	(11,399)		(11,602)
Balance at beginning of year	26,831		26,547		25,875
Acquistion of subsidiaries	320		86		14
Total	27,151		26,633		25,889
Net losses and loss expenses incurred in respect of losses incurring in Current Year	10,176		9,878		10,132
Net losses and loss expenses incurred in respect of losses incurring in Prior Year	(527)		(530)		(479)
Total	9,649		9,348		9,653
Net losses and loss expenses incurred in respect of losses paid in Current Year	3,975		3,942		4,325
Net losses and loss expenses incurred in respect of losses paid in Prior Year	5,260		5,035		4,894
Total	9,235		8,977		9,219
Foreign currency revaluation and other	(557)		(173)		224
Balance at end of year	27,008		26,831		26,547
Reinsurance recoverable on unpaid losses	(11,307)	[1]	(10,612)	[1]	(11,399)
Gross unpaid losses and loss expenses, end of year	$ 38,315		$ 37,443		$ 37,946

[1] (1) Net of a provision for uncollectible reinsurance.