Investments (Ratings distribution of asset-backed securities in a gross unrealized loss position) (Details) (Asset-backed Securities [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Asset-Backed Securities
Amortized Cost	$ 1,860
Estimated Fair Value	1,691	2,582
Gross Unrealized Losses	169	246
US Government, Government Agencies, and Government-Sponsored Enterprises [Member]
Asset-Backed Securities
Amortized Cost	468
Estimated Fair Value	405
Gross Unrealized Losses	63
AAA [Member]
Asset-Backed Securities
Amortized Cost	247
Estimated Fair Value	241
Gross Unrealized Losses	6
AA [Member]
Asset-Backed Securities
Amortized Cost	163
Estimated Fair Value	155
Gross Unrealized Losses	8
A [Member]
Asset-Backed Securities
Amortized Cost	141
Estimated Fair Value	134
Gross Unrealized Losses	7
BBB [Member]
Asset-Backed Securities
Amortized Cost	162
Estimated Fair Value	148
Gross Unrealized Losses	14
Non-investment grade [Member]
Asset-Backed Securities
Amortized Cost	679
Estimated Fair Value	608
Gross Unrealized Losses	$ 71