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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of period				$ 9,741				$ 9,678	$ 9,741		$ 9,678	$ 8,563
Less reinsurance recoverables, net of allowance	$ 3,024				$ 2,942				3,024		2,942	2,957		$ 2,302
Net liability at beginning of period				6,799				6,721	6,799		6,721	6,261
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current year									3,414		3,195	3,019
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	45	$ 12	$ 41	$ 45	61	$ 31	$ 44	$ 56	143		192
Total losses and LAE incurred									3,271		3,003	2,955
Payments for losses and LAE of:
Current year									(1,076)		(963)	(942)
Prior years									(1,790)		(1,639)	(1,586)
Total payments									(2,866)		(2,602)	(2,528)
Reserves of businesses disposed									0	[1]	(319)	0	[1]
Foreign currency translation and other									4		(4)	33
Net liability at end of period	7,208				6,799				7,208		6,799	6,721
Add back reinsurance recoverables, net of allowance	3,024				2,942				3,024		2,942	2,957
Gross unpaid losses and LAE included in the balance sheet	$ 10,232				$ 9,741				10,232		9,741	9,678
Special A&E charges
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Special A&E charges									18		18	89
Neon exited lines charge
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									7		0	(18)
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									$ (168)		$ (210)	$ (135)

[1] Reflects the reinsurance to close transactions at Neon (discussed below).