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	Dec. 31, 2017	Dec. 31, 2016
Debt Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	$ 11,817	$ 14,089
Less Than 12 Months, Gross Unrealized Losses	(67)	(235)
Greater Than 12 Months, Fair Value	4,441	313
Greater Than 12 Months, Gross Unrealized Losses	(86)	(8)
Total, Fair Value	16,258	14,402
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(153)	(243)
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	1,249	1,794
Less Than 12 Months, Gross Unrealized Losses	(8)	(31)
Greater Than 12 Months, Fair Value	1,027
Greater Than 12 Months, Gross Unrealized Losses	(22)
Total, Fair Value	2,276	1,794
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(30)	(31)
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	2,599	4,376
Less Than 12 Months, Gross Unrealized Losses	(21)	(101)
Greater Than 12 Months, Fair Value	866
Greater Than 12 Months, Gross Unrealized Losses	(14)
Total, Fair Value	3,465	4,376
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(35)	(101)
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	5,901	5,128
Less Than 12 Months, Gross Unrealized Losses	(23)	(56)
Greater Than 12 Months, Fair Value	1,242	137
Greater Than 12 Months, Gross Unrealized Losses	(21)	(2)
Total, Fair Value	7,143	5,265
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(44)	(58)
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	1,657	2,247
Less Than 12 Months, Gross Unrealized Losses	(12)	(40)
Greater Than 12 Months, Fair Value	1,162	79
Greater Than 12 Months, Gross Unrealized Losses	(26)	(3)
Total, Fair Value	2,819	2,326
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(38)	(43)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	411	544
Less Than 12 Months, Gross Unrealized Losses	(3)	(7)
Greater Than 12 Months, Fair Value	144	97
Greater Than 12 Months, Gross Unrealized Losses	(3)	(3)
Total, Fair Value	555	641
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(6)	(10)
Equity Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	97	93
Less Than 12 Months, Gross Unrealized Losses	(5)	(5)
Greater Than 12 Months, Fair Value	105	91
Greater Than 12 Months, Gross Unrealized Losses	(36)	(42)
Total, Fair Value	202	184
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (41)	$ (47)