Claim and Claim Adjustment Expense Reserves - Reconciliation of claim and claim adjustment expense reserves (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 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	3,949	3,799
Decrease in provision for insured events of prior years	(284)	(332)
Amortization of discount	138	134
Total net incurred	3,803	3,601
Net payments attributable to:
Current year events	(560)	(591)
Prior year events	(3,401)	(3,209)
Total net payments	(3,961)	(3,800)
Foreign currency translation adjustment and other	110	39
Net reserves, end of period	18,201	18,416
Ceded reserves, end of year	4,008	4,256
Gross reserves, end of year	$ 22,209	$ 22,672