Insurance - Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of year			$ 10,232	$ 9,741
Less reinsurance recoverables, net of allowance			3,024	2,942
Net liability at beginning of year			7,208	6,799
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current period			2,560	2,457
Net increase (decrease) in the provision for claims of prior years			(166)	(116)
Total losses and LAE incurred			2,441	2,359
Payments for losses and LAE of:
Current year			(592)	(731)
Prior years			(1,406)	(1,408)
Total payments			(1,998)	(2,139)
Foreign currency translation and other			(11)	(5)
Net liability at end of period	$ 7,640	$ 7,014	7,640	7,014
Add back reinsurance recoverables, net of allowance	3,114	2,833	3,114	2,833
Gross unpaid losses and LAE included in the balance sheet at end of period	10,754	9,847	10,754	9,847
Property and Casualty Insurance | Other lines
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Special A&E charges	$ 47	$ 18	$ 47	$ 18