Insurance - Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 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	2,237	2,011
Net increase (decrease) in the provision for claims of prior years:
Total losses and LAE incurred	2,239	2,033
Payments for losses and LAE of:
Current year	(530)	(463)
Prior years	(1,272)	(1,206)
Total payments	(1,802)	(1,669)
Foreign currency translation and other	32	(3)
Net liability at end of period	6,730	6,287
Add back reinsurance recoverables, net of allowance	2,833	2,374
Gross unpaid losses and LAE included in the balance sheet at end of period	9,563	8,661
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 and other	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 and other	0	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 and other	$ (87)	$ (71)