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	Mar. 31, 2020	Dec. 31, 2019
Debt Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	$ 11,001	$ 4,022
Less Than 12 Months, Gross Unrealized Losses	(436)	(22)
Greater Than 12 Months, Fair Value	421	1,073
Greater Than 12 Months, Gross Unrealized Losses	(32)	(7)
Total, Fair Value	11,422	5,095
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(468)	(29)
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	56	616
Less Than 12 Months, Gross Unrealized Losses	(1)	(4)
Greater Than 12 Months, Fair Value	0	0
Greater Than 12 Months, Gross Unrealized Losses	0	0
Total, Fair Value	56	616
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1)	(4)
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	810	440
Less Than 12 Months, Gross Unrealized Losses	(20)	(5)
Greater Than 12 Months, Fair Value	0	0
Greater Than 12 Months, Gross Unrealized Losses	0	0
Total, Fair Value	810	440
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(20)	(5)
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	9,294	1,903
Less Than 12 Months, Gross Unrealized Losses	(382)	(7)
Greater Than 12 Months, Fair Value	391	740
Greater Than 12 Months, Gross Unrealized Losses	(29)	(4)
Total, Fair Value	9,685	2,643
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(411)	(11)
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value		657
Less Than 12 Months, Gross Unrealized Losses		(3)
Greater Than 12 Months, Fair Value		333
Greater Than 12 Months, Gross Unrealized Losses		(3)
Total, Fair Value		990
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		(6)
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less Than 12 Months, Fair Value	841	406
Less Than 12 Months, Gross Unrealized Losses	(33)	(3)
Greater Than 12 Months, Fair Value	30	0
Greater Than 12 Months, Gross Unrealized Losses	(3)	0
Total, Fair Value	871	406
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (36)	$ (3)