Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 15,355
Provision	$ 321	$ 300
% of Gross	2.10%
Largest reinsurers
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 5,190
Provision	$ 59
% of Gross	1.10%
Other reinsurers balances rated A- or better
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 5,898
Provision	$ 58
% of Gross	1.00%
Other reinsurers balances with ratings lower than A- or not rated
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 681
Provision	$ 75
% of Gross	11.00%
Other pools and government agencies
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 577
Provision	$ 15
% of Gross	2.60%
Structured settlements
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 550
Provision	$ 16
% of Gross	2.90%
Other captives
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 2,199
Provision	$ 18
% of Gross	0.80%
Other
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 260
Provision	$ 80
% of Gross	30.80%