Claim and Claim Adjustment Expense Reserves (Liability for Unpaid Claim and Claim Adjustment Expense Rollforward) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Abstract]
Gross, beginning of year	$ 22,004	$ 22,343
Ceded, beginning of year	3,934	4,094
Net reserves, beginning of year	18,070	18,249
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	3,866	3,949
Decrease in provision for insured events of prior years	(173)	(284)
Amortization of discount	136	138
Total net incurred	3,829	3,803
Net payments attributable to:
Current year events	(658)	(560)
Prior year events	(3,415)	(3,401)
Total net payments	(4,073)	(3,961)
Foreign currency translation adjustment and other	(80)	110
Net reserves, end of period	17,746	18,201
Ceded reserves, end of period	3,858	4,008
Gross reserves, end of period	$ 21,604	$ 22,209