Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year		$ 63,179	$ 60,540	$ 37,303
Reinsurance recoverable on unpaid losses, beginning of year	[1]	(14,014)	(12,708)	(10,741)
Net unpaid losses and loss expenses, beginning of year		49,165	47,832	26,562
Acquistion of subsidiaries		0	0	21,402
Total		49,165	47,832	47,964
Net losses and loss expenses incurred in respect of losses incurring in Current Year		19,048	19,391	17,256
Prior Year Claims and Claims Adjustment Expense	[2]	(981)	(937)	(1,204)
Total		18,067	18,454	16,052
Net losses and loss expenses paid in Current Year		7,544	6,575	5,899
Net losses and loss expenses paid in Prior Year		10,796	10,873	9,816
Total		18,340	17,448	15,715
Foreign currency revaluation and other		(621)	327	(469)
Net unpaid losses and loss expenses, end of year		48,271	49,165	47,832
Reinsurance recoverable on unpaid losses, end of year	[1]	14,689	14,014	12,708
Gross unpaid losses and loss expenses, end of year		62,960	63,179	60,540
prior period development, net adjustments		$ 85	$ 108	$ 69

[1] Net of provision for uncollectible reinsurance.

[2] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $85 million, $108 million and $69 million for 2018, 2017, and 2016, respectively.