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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of period				$ 9,678				$ 8,563	$ 9,678		$ 8,563	$ 8,127
Less reinsurance recoverables, net of allowance	$ 2,942				$ 2,957				2,942		2,957	2,302		$ 2,201
Net liability at beginning of period				6,721				6,261	6,721		6,261	5,926
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current year									3,195		3,019	2,730
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	(61)	$ (31)	$ (44)	$ (56)	(66)	$ 52	$ (22)	$ (28)	(192)		(64)
Total losses and LAE incurred									3,003		2,955	2,762
Payments for losses and LAE of:
Current year									(963)		(942)	(841)
Prior years									(1,639)		(1,586)	(1,512)
Total payments									(2,602)		(2,528)	(2,353)
Reserves of businesses disposed									(319)	[1]	0	(40)	[1]
Foreign currency translation and other									(4)		33	(34)
Net liability at end of period	6,799				6,721				6,799		6,721	6,261
Add back reinsurance recoverables, net of allowance	2,942				2,957				2,942		2,957	2,302
Gross unpaid losses and LAE included in the balance sheet	$ 9,741				$ 9,678				9,741		9,678	8,563
Special A&E charges
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									18		89	36
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									0		(18)	57
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									$ (210)		$ (135)	$ (61)

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