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, 2019	Dec. 31, 2018
Debt Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	$ 2,729	$ 12,355
Less Than 12 Months, Gross Unrealized Losses	(11)	(156)
Greater Than 12 Months, Fair Value	1,851	11,002
Greater Than 12 Months, Gross Unrealized Losses	(10)	(287)
Total, Fair Value	4,580	23,357
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(21)	(443)
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	286	998
Less Than 12 Months, Gross Unrealized Losses	(1)	(7)
Greater Than 12 Months, Fair Value	228	1,425
Greater Than 12 Months, Gross Unrealized Losses	(1)	(35)
Total, Fair Value	514	2,423
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2)	(42)
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	296	1,334
Less Than 12 Months, Gross Unrealized Losses	(2)	(11)
Greater Than 12 Months, Fair Value	83	2,491
Greater Than 12 Months, Gross Unrealized Losses	(1)	(46)
Total, Fair Value	379	3,825
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3)	(57)
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	1,360	8,105
Less Than 12 Months, Gross Unrealized Losses	(5)	(109)
Greater Than 12 Months, Fair Value	1,022	4,239
Greater Than 12 Months, Gross Unrealized Losses	(4)	(109)
Total, Fair Value	2,382	12,344
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9)	(218)
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	570	1,296
Less Than 12 Months, Gross Unrealized Losses	(2)	(22)
Greater Than 12 Months, Fair Value	518	2,388
Greater Than 12 Months, Gross Unrealized Losses	(4)	(84)
Total, Fair Value	1,088	3,684
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(6)	(106)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	217	622
Less Than 12 Months, Gross Unrealized Losses	(1)	(7)
Greater Than 12 Months, Fair Value	0	459
Greater Than 12 Months, Gross Unrealized Losses	0	(13)
Total, Fair Value	217	1,081
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1)	$ (20)