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	Jun. 30, 2017	Dec. 31, 2016
Debt Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	$ 11,401	$ 14,089
Less Than 12 Months, Gross Unrealized Losses	(114)	(235)
Greater Than 12 Months, Fair Value	128	313
Greater Than 12 Months, Gross Unrealized Losses	(5)	(8)
Total, Fair Value	11,529	14,402
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(119)	(243)
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	2,079	1,794
Less Than 12 Months, Gross Unrealized Losses	(23)	(31)
Total, Fair Value	2,079	1,794
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(23)	(31)
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	2,441	4,376
Less Than 12 Months, Gross Unrealized Losses	(27)	(101)
Total, Fair Value	2,441	4,376
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(27)	(101)
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	4,294	5,128
Less Than 12 Months, Gross Unrealized Losses	(28)	(56)
Greater Than 12 Months, Fair Value	43	137
Greater Than 12 Months, Gross Unrealized Losses	(2)	(2)
Total, Fair Value	4,337	5,265
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(30)	(58)
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	2,159	2,247
Less Than 12 Months, Gross Unrealized Losses	(32)	(40)
Greater Than 12 Months, Fair Value	72	79
Greater Than 12 Months, Gross Unrealized Losses	(2)	(3)
Total, Fair Value	2,231	2,326
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(34)	(43)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	428	544
Less Than 12 Months, Gross Unrealized Losses	(4)	(7)
Greater Than 12 Months, Fair Value	13	97
Greater Than 12 Months, Gross Unrealized Losses	(1)	(3)
Total, Fair Value	441	641
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5)	(10)
Equity Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	73	93
Less Than 12 Months, Gross Unrealized Losses	(4)	(5)
Greater Than 12 Months, Fair Value	100	91
Greater Than 12 Months, Gross Unrealized Losses	(36)	(42)
Total, Fair Value	173	184
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (40)	$ (47)