Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year		$ 62,960	$ 63,179	$ 60,540
Reinsurance recoverable on unpaid losses, beginning of year	[1]	(14,689)	(14,014)	(12,708)
Net unpaid losses and loss expenses, beginning of year		48,271	49,165	47,832
Net losses and loss expenses incurred in respect of losses incurring in Current Year		19,575	19,048	19,391
Prior Year Claims and Claims Adjustment Expense	[2]	(845)	(981)	(937)
Total		18,730	18,067	18,454
Net losses and loss expenses paid in Current Year		7,894	7,544	6,575
Net losses and loss expenses paid in Prior Year		10,579	10,796	10,873
Total		18,473	18,340	17,448
Foreign currency revaluation and other		(19)	(621)	327
Net unpaid losses and loss expenses, end of year		48,509	48,271	49,165
Reinsurance recoverable on unpaid losses, end of year	[1]	14,181	14,689	14,014
Gross unpaid losses and loss expenses, end of year		62,690	62,960	63,179
Prior Period Development, net adjustments		$ 53	$ 85	$ 108

[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 $53 million, $85 million and $108 million for 2019, 2018, and 2017, respectively.