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	Sep. 30, 2015	Dec. 31, 2014
Debt Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	$ 3,988	$ 3,980
Less Than 12 Months, Gross Unrealized Losses	(50)	(24)
Greater Than 12 Months, Fair Value	502	949
Greater Than 12 Months, Gross Unrealized Losses	(13)	(17)
Total, Fair Value	4,490	4,929
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(63)	(41)
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value		420
Less Than 12 Months, Gross Unrealized Losses		(1)
Total, Fair Value		420
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		(1)
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	507	711
Less Than 12 Months, Gross Unrealized Losses	(5)	(4)
Greater Than 12 Months, Fair Value	17	99
Greater Than 12 Months, Gross Unrealized Losses	(1)	(1)
Total, Fair Value	524	810
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(6)	(5)
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	2,799	2,595
Less Than 12 Months, Gross Unrealized Losses	(39)	(17)
Greater Than 12 Months, Fair Value	270	464
Greater Than 12 Months, Gross Unrealized Losses	(8)	(9)
Total, Fair Value	3,069	3,059
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(47)	(26)
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	365
Less Than 12 Months, Gross Unrealized Losses	(3)
Greater Than 12 Months, Fair Value	122	272
Greater Than 12 Months, Gross Unrealized Losses	(3)	(5)
Total, Fair Value	487	272
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(6)	(5)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	317	254
Less Than 12 Months, Gross Unrealized Losses	(3)	(2)
Greater Than 12 Months, Fair Value	93	114
Greater Than 12 Months, Gross Unrealized Losses	(1)	(2)
Total, Fair Value	410	368
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4)	(4)
Equity Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	140	107
Less Than 12 Months, Gross Unrealized Losses	(16)	(6)
Greater Than 12 Months, Fair Value	82	88
Greater Than 12 Months, Gross Unrealized Losses	(42)	(19)
Total, Fair Value	222	195
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (58)	$ (25)