Insurance - Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of period				$ 10,232				$ 9,741	$ 10,232	$ 9,741	$ 9,678
Less reinsurance recoverables, net of allowance	$ 3,117				$ 3,024				3,117	3,024	2,942	$ 2,957
Net liability at beginning of period				7,208				6,799	7,208	6,799	6,721
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current year									3,398	3,414	3,195
Net increase (decrease) in the provision for claims of prior years:
Provision for claims of prior years related to special A&E charges, Neon exited lines development and other	8	$ 0	$ 77	$ 42	45	$ 12	$ 41	$ 45	127	143
Total losses and LAE incurred									3,271	3,271	3,003
Payments for losses and LAE of:
Current year									(990)	(1,076)	(963)
Prior years									(1,766)	(1,790)	(1,639)
Total payments									(2,756)	(2,866)	(2,602)
Reserves of businesses disposed									(449)	0	(319)
Foreign currency translation and other									1	4	(4)
Net liability at end of period	7,275				7,208				7,275	7,208	6,799
Add back reinsurance recoverables, net of allowance	3,117				3,024				3,117	3,024	2,942
Gross unpaid losses and LAE included in the balance sheet	$ 10,392				$ 10,232				10,392	10,232	9,741
Special A&E charges
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Special A&E charges									47	18	18
Neon exited lines
Net increase (decrease) in the provision for claims of prior years:
Provision for claims of prior years related to special A&E charges, Neon exited lines development and other									19	7	0
Other
Net increase (decrease) in the provision for claims of prior years:
Provision for claims of prior years related to special A&E charges, Neon exited lines development and other									$ (193)	$ (168)	$ (210)