Claim and Claim Adjustment Expense Reserves (Liability for Unpaid Claim and Claim Adjustment Expense Rollforward) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Reserves, beginning of year:
Gross		$ 22,004	$ 22,343
Ceded		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		2,552	2,443
Decrease in provision for insured events of prior years		(112)	(159)
Amortization of discount		92	93
Total net incurred	[1]	2,532	2,377
Net payments attributable to:
Current year events		(312)	(266)
Prior year events		(2,387)	(2,331)
Total net payments		(2,699)	(2,597)
Foreign currency translation adjustment and other		(70)	70
Net reserves, end of period		17,833	18,099
Ceded reserves, end of period		4,157	4,080
Gross reserves, end of period		$ 21,990	$ 22,179

[1] Total net incurred above does not agree to Insurance claims and policyholders' benefits as reflected on the Condensed Consolidated Statements of Operations due to amounts related to retroactive reinsurance deferred gain accounting, uncollectible reinsurance and loss deductible receivables, and benefit expenses related to future policy benefits which are not reflected in the table above.