Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	$ 12,517
Provision	439	479
% of Gross	3.50%
Largest reinsurers
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	5,800
Provision	98
% of Gross	1.70%
Other reinsurers balances rated A- or better
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	3,080
Provision	40
% of Gross	1.30%
Other reinsurers balances with ratings lower than A- or not rated
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	602
Provision	120
% of Gross	19.90%
Other pools and government agencies
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	383
Provision	14
% of Gross	3.70%
Structured settlements
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	582
Provision	24
% of Gross	4.10%
Other captives
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	1,761
Provision	14
% of Gross	0.80%
Other
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
2012	309
Provision	$ 129
% of Gross	41.70%