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, 2019	Dec. 31, 2018
Debt Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	$ 703	$ 12,355
Less Than 12 Months, Gross Unrealized Losses	(3)	(156)
Greater Than 12 Months, Fair Value	5,063	11,002
Greater Than 12 Months, Gross Unrealized Losses	(31)	(287)
Total, Fair Value	5,766	23,357
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(34)	(443)
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	0	998
Less Than 12 Months, Gross Unrealized Losses	0	(7)
Greater Than 12 Months, Fair Value	655	1,425
Greater Than 12 Months, Gross Unrealized Losses	(5)	(35)
Total, Fair Value	655	2,423
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5)	(42)
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	0	1,334
Less Than 12 Months, Gross Unrealized Losses	0	(11)
Greater Than 12 Months, Fair Value	366	2,491
Greater Than 12 Months, Gross Unrealized Losses	(1)	(46)
Total, Fair Value	366	3,825
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1)	(57)
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	703	8,105
Less Than 12 Months, Gross Unrealized Losses	(3)	(109)
Greater Than 12 Months, Fair Value	2,352	4,239
Greater Than 12 Months, Gross Unrealized Losses	(9)	(109)
Total, Fair Value	3,055	12,344
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(12)	(218)
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	0	1,296
Less Than 12 Months, Gross Unrealized Losses	0	(22)
Greater Than 12 Months, Fair Value	1,562	2,388
Greater Than 12 Months, Gross Unrealized Losses	(15)	(84)
Total, Fair Value	1,562	3,684
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(15)	(106)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	0	622
Less Than 12 Months, Gross Unrealized Losses	0	(7)
Greater Than 12 Months, Fair Value	128	459
Greater Than 12 Months, Gross Unrealized Losses	(1)	(13)
Total, Fair Value	128	1,081
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1)	$ (20)