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, 2014	Dec. 31, 2013
Debt Securities [Member]
Less Than 12 Months, Fair Value	$ 5,969	$ 7,801
Less Than 12 Months, Gross Unrealized Losses	(100)	(189)
Greater Than 12 Months, Fair Value	394	379
Greater Than 12 Months, Gross Unrealized Losses	(24)	(32)
Total, Fair Value	6,363	8,180
Total, Unrealized Losses	(124)	(221)
U.S. Government and Agency Obligations [Member]
Less Than 12 Months, Fair Value	796	1,055
Less Than 12 Months, Gross Unrealized Losses	(13)	(19)
Greater Than 12 Months, Fair Value	11	17
Greater Than 12 Months, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	807	1,072
Total, Unrealized Losses	(14)	(21)
State and Municipal Obligations [Member]
Less Than 12 Months, Fair Value	1,564	2,491
Less Than 12 Months, Gross Unrealized Losses	(24)	(62)
Greater Than 12 Months, Fair Value	128	128
Greater Than 12 Months, Gross Unrealized Losses	(6)	(10)
Total, Fair Value	1,692	2,619
Total, Unrealized Losses	(30)	(72)
Corporate Obligations [Member]
Less Than 12 Months, Fair Value	2,174	2,573
Less Than 12 Months, Gross Unrealized Losses	(26)	(51)
Greater Than 12 Months, Fair Value	116	103
Greater Than 12 Months, Gross Unrealized Losses	(7)	(9)
Total, Fair Value	2,290	2,676
Total, Unrealized Losses	(33)	(60)
U.S. Agency Mortgage-Backed Securities [Member]
Less Than 12 Months, Fair Value	1,146	1,393
Less Than 12 Months, Gross Unrealized Losses	(33)	(51)
Greater Than 12 Months, Fair Value	108	105
Greater Than 12 Months, Gross Unrealized Losses	(9)	(10)
Total, Fair Value	1,254	1,498
Total, Unrealized Losses	(42)	(61)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Less Than 12 Months, Fair Value	289	289
Less Than 12 Months, Gross Unrealized Losses	(4)	(6)
Greater Than 12 Months, Fair Value	31	26
Greater Than 12 Months, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	320	315
Total, Unrealized Losses	(5)	(7)
Equity Securities [Member]
Less Than 12 Months, Fair Value	167	180
Less Than 12 Months, Gross Unrealized Losses	(15)	(13)
Greater Than 12 Months, Fair Value	0
Greater Than 12 Months, Gross Unrealized Losses	0
Total, Fair Value	167	180
Total, Unrealized Losses	$ (15)	$ (13)