Investments (Ratings distribution of asset-backed securities in a gross unrealized loss position) (Details) (Asset-backed Securities [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Asset-Backed Securities
Amortized Cost	$ 2,828
Estimated Fair Value	2,582	4,280
Gross Unrealized Losses	246	314
US Government, Government Agencies, and Government-Sponsored Enterprises [Member]
Asset-Backed Securities
Amortized Cost	382
Estimated Fair Value	347
Gross Unrealized Losses	35
AAA [Member]
Asset-Backed Securities
Amortized Cost	364
Estimated Fair Value	355
Gross Unrealized Losses	9
AA [Member]
Asset-Backed Securities
Amortized Cost	409
Estimated Fair Value	388
Gross Unrealized Losses	21
A [Member]
Asset-Backed Securities
Amortized Cost	370
Estimated Fair Value	357
Gross Unrealized Losses	13
BBB [Member]
Asset-Backed Securities
Amortized Cost	319
Estimated Fair Value	294
Gross Unrealized Losses	25
Non-investment grade [Member]
Asset-Backed Securities
Amortized Cost	984
Estimated Fair Value	841
Gross Unrealized Losses	$ 143