Investments (Ratings distribution of corporate and other bonds in a gross unrealized loss position) (Details) (Corporate and other bonds [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Corporate and Other Bonds
Amortized Cost	$ 2,865
Estimated Fair Value	2,711	2,124
Gross Unrealized Losses	154	70
AAA [Member]
Corporate and Other Bonds
Amortized Cost	112
Estimated Fair Value	111
Gross Unrealized Losses	1
AA [Member]
Corporate and Other Bonds
Amortized Cost	97
Estimated Fair Value	94
Gross Unrealized Losses	3
A [Member]
Corporate and Other Bonds
Amortized Cost	895
Estimated Fair Value	853
Gross Unrealized Losses	42
BBB [Member]
Corporate and Other Bonds
Amortized Cost	1,275
Estimated Fair Value	1,196
Gross Unrealized Losses	79
Non-investment grade [Member]
Corporate and Other Bonds
Amortized Cost	486
Estimated Fair Value	457
Gross Unrealized Losses	$ 29