Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 13,877
Provision	$ 300	$ 328
% of Gross	2.20%
Largest reinsurers
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 5,064
Provision	$ 59
% of Gross	1.20%
Other reinsurers balances rated A- or better
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 4,699
Provision	$ 52
% of Gross	1.10%
Other reinsurers balances with ratings lower than A- or not rated
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 586
Provision	$ 70
% of Gross	11.90%
Other pools and government agencies
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 577
Provision	$ 11
% of Gross	1.90%
Structured settlements
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 557
Provision	$ 14
% of Gross	2.50%
Other captives
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 2,172
Provision	$ 16
% of Gross	0.70%
Other
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross reinsurance recoverable	$ 222
Provision	$ 78
% of Gross	35.10%