Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year	$ 38,315		$ 37,443		$ 37,946
Reinsurance recoverable on unpaid losses	(11,307)	[1]	(10,612)		(11,399)
Balance at beginning of year	27,008		26,831		26,547
Acquistion of subsidiaries	417		320		86
Total	27,425		27,151		26,633
Net losses and loss expenses incurred in respect of losses incurring in Current Year	10,030		10,176		9,878
Net losses and loss expenses incurred in prior years	(546)		(527)		(530)
Total	9,484		9,649		9,348
Net losses and loss expenses incurred in respect of losses paid in Current Year	4,053		3,975		3,942
Net losses and loss expenses incurred in respect of losses paid in Prior Year	5,612		5,260		5,035
Total	9,665		9,235		8,977
Foreign currency revaluation and other	(682)		(557)		(173)
Balance at end of year	26,562		27,008		26,831
Reinsurance recoverable on unpaid losses	(10,741)	[1]	(11,307)	[1]	(10,612)
Gross unpaid losses and loss expenses, end of year	$ 37,303		$ 38,315		$ 37,443

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