Insurance - Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of year		$ 9,678	$ 8,563
Less reinsurance recoverables, net of allowance		2,957	2,302
Net liability at beginning of year		6,721	6,261
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current period		1,434	1,294
Total losses and LAE incurred		1,334	1,244
Payments for losses and LAE of:
Current year		(294)	(253)
Prior years		(975)	(953)
Total payments		(1,269)	(1,206)
Reserves of business disposed	[1]	(319)	0
Foreign currency translation and other		(4)	24
Net liability at end of period		6,463	6,323
Add back reinsurance recoverables, net of allowance		2,630	2,407
Gross unpaid losses and LAE included in the balance sheet at end of period		9,093	8,730
Net increase (decrease) in the provision for claims of prior years
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Net increase (decrease) in the provision for claims of prior years		$ (100)	$ (50)

[1] Reflects the reinsurance to close transaction at Neon discussed below.