Claim and Claim Adjustment Expense Reserves - Reconciliation of claim and claim adjustment expense reserves (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	1,207	1,224
Decrease in provision for insured events of prior years	(82)	(45)
Amortization of discount	48	48
Total net incurred	1,173	1,227
Net payments attributable to:
Current year events	(68)	(76)
Prior year events	(1,184)	(1,147)
Total net payments	(1,252)	(1,223)
Foreign currency translation adjustment and other	14	39
Net reserves, end of period	18,184	18,619
Ceded reserves, end of year	4,076	4,399
Gross reserves, end of year	$ 22,260	$ 23,018