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, 2017	Jun. 30, 2016
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of year	$ 8,563	$ 8,127
Less reinsurance recoverables, net of allowance	2,302	2,201
Net liability at beginning of year	6,261	5,926
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current period	1,294	1,268
Net increase (decrease) in the provision for claims of prior years:
Net increase (decrease) in the provision for claims of prior years	(50)	0
Total losses and LAE incurred	1,244	1,268
Payments for losses and LAE of:
Current year	(253)	(245)
Prior years	(953)	(888)
Total payments	(1,206)	(1,133)
Foreign currency translation and other	24	1
Net liability at end of period	6,323	6,062
Add back reinsurance recoverables, net of allowance	2,407	2,141
Gross unpaid losses and LAE included in the balance sheet at end of period	$ 8,730	$ 8,203