Claim and Claim Adjustment Expense Reserves - Reconciliation of claim and claim adjustment expense reserves (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross, beginning of year	$ 22,343	$ 22,663
Ceded, beginning of year	4,094	4,087
Net reserves, beginning of year	18,249	18,576
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	2,443	2,583
Decrease in provision for insured events of prior years	(159)	(198)
Amortization of discount	93	93
Total net incurred	2,377	2,478
Net payments attributable to:
Current year events	(266)	(311)
Prior year events	(2,331)	(2,185)
Total net payments	(2,597)	(2,496)
Foreign currency translation adjustment and other	70	46
Net reserves, end of period	18,099	18,604
Ceded reserves, end of year	4,080	4,371
Gross reserves, end of year	$ 22,179	$ 22,975