Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year	$ 37,303	$ 38,315	$ 37,443
Reinsurance recoverable on unpaid losses	(10,741)	(11,307)	(10,612)
Balance at beginning of year	26,562	27,008	26,831
Acquistion of subsidiaries	21,402	417	320
Total	47,964	27,425	27,151
Net losses and loss expenses incurred in respect of losses incurring in Current Year	17,256	10,030	10,176
Prior Year Claims and Claims Adjustment Expense	(1,204)	(546)	(527)
Total	16,052	9,484	9,649
Net losses and loss expenses incurred in respect of losses paid in Current Year	5,899	4,053	3,975
Net losses and loss expenses incurred in respect of losses paid in Prior Year	9,816	5,612	5,260
Total	15,715	9,665	9,235
Foreign currency revaluation and other	469	682	557
Balance at end of year	47,832	26,562	27,008
Reinsurance recoverable on unpaid losses	(12,708)	(10,741)	(11,307)
Gross unpaid losses and loss expenses, end of year	$ 60,540	$ 37,303	$ 38,315