Claim and Claim Adjustment Expense Reserves (International - Short Duration Contracts) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,070	$ 18,249	$ 18,576	$ 18,927
International
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,432	$ 1,201
Excluding Hardy | International
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,026
Hardy | International
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 406