Investments (Ratings distribution of asset-backed securities in a gross unrealized loss position) (Details) (Asset-backed Securities [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Asset-Backed Securities
Amortized Cost	$ 2,722
Estimated Fair Value	2,546	2,582
Gross Unrealized Losses	176	246
US Government, Government Agencies, and Government-Sponsored Enterprises [Member]
Asset-Backed Securities
Amortized Cost	852
Estimated Fair Value	810
Gross Unrealized Losses	42
AAA [Member]
Asset-Backed Securities
Amortized Cost	246
Estimated Fair Value	239
Gross Unrealized Losses	7
AA [Member]
Asset-Backed Securities
Amortized Cost	226
Estimated Fair Value	215
Gross Unrealized Losses	11
A [Member]
Asset-Backed Securities
Amortized Cost	294
Estimated Fair Value	286
Gross Unrealized Losses	8
BBB [Member]
Asset-Backed Securities
Amortized Cost	209
Estimated Fair Value	193
Gross Unrealized Losses	16
Non-investment grade [Member]
Asset-Backed Securities
Amortized Cost	895
Estimated Fair Value	803
Gross Unrealized Losses	$ 92