Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 16,316
Provision	$ 323	$ 321
% of Gross	2.00%
Largest reinsurers
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 6,578
Provision	$ 70
% of Gross	1.10%
Other reinsurers balances rated A- or better
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 5,339
Provision	$ 63
% of Gross	1.20%
Other reinsurers balances with ratings lower than A- or not rated
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 558
Provision	$ 68
% of Gross	12.20%
Other pools and government agencies
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 429
Provision	$ 16
% of Gross	3.70%
Structured settlements
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 548
Provision	$ 18
% of Gross	3.30%
Other captives
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 2,590
Provision	$ 16
% of Gross	0.60%
Other
Ceded Credit Risk [Line Items]
Gross reinsurance recoverable	$ 274
Provision	$ 72
% of Gross	26.30%