Investments (Fair Value of Available-For-Sale Investments with Gross Unrealized Losses by Investment Type and Length of Time That Individual Securities Have Been in a Continuous Unrealized Loss Position) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Debt Securities [Member]
Less Than 12 Months, Fair Value	$ 2,256	$ 1,240
Less Than 12 Months, Gross Unrealized Losses	(17)	(8)
12 Months or Greater, Fair Value
12 Months or Greater, Gross Unrealized Losses
Total, Fair Value	2,256	1,240
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(17)	(8)
US Treasury and Government [Member]
Less Than 12 Months, Fair Value	142	183
Less Than 12 Months, Gross Unrealized Losses	(1)	(1)
12 Months or Greater, Fair Value
12 Months or Greater, Gross Unrealized Losses
Total, Fair Value	142	183
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(1)	(1)
State and Municipal Obligations [Member]
Less Than 12 Months, Fair Value	417	362
Less Than 12 Months, Gross Unrealized Losses	(5)	(3)
12 Months or Greater, Fair Value
12 Months or Greater, Gross Unrealized Losses
Total, Fair Value	417	362
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(5)	(3)
Corporate Obligations [Member]
Less Than 12 Months, Fair Value	1,120	695
Less Than 12 Months, Gross Unrealized Losses	(6)	(4)
12 Months or Greater, Fair Value
12 Months or Greater, Gross Unrealized Losses
Total, Fair Value	1,120	695
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(6)	(4)
U.S. Agency Mortgage-Backed Securities [Member]
Less Than 12 Months, Fair Value	420
Less Than 12 Months, Gross Unrealized Losses	(4)
12 Months or Greater, Fair Value
12 Months or Greater, Gross Unrealized Losses
Total, Fair Value	420
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(4)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Less Than 12 Months, Fair Value	157
Less Than 12 Months, Gross Unrealized Losses	(1)
12 Months or Greater, Fair Value
12 Months or Greater, Gross Unrealized Losses
Total, Fair Value	157
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(1)
Equity Securities [Member]
Less Than 12 Months, Fair Value	40	13
Less Than 12 Months, Gross Unrealized Losses	(1)	(1)
12 Months or Greater, Fair Value	2
12 Months or Greater, Gross Unrealized Losses	(1)
Total, Fair Value	42	13
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ (2)	$ (1)