Financial Instruments (Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 13,227	$ 12,953
Less than 12 Months, Unrealized Loss	(27)	(35)
12 Months or Greater, Fair Value	273	8,403
12 Months or Greater, Unrealized Loss	(5)	(39)
Total Fair Value	13,500	21,356
Total Unrealized Loss	(32)	(74)
Government bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	5,473	6,752
Less than 12 Months, Unrealized Loss	(9)	(20)
12 Months or Greater, Fair Value	0	4,590
12 Months or Greater, Unrealized Loss	0	(10)
Total Fair Value	5,473	11,342
Total Unrealized Loss	(9)	(30)
Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	5,764	1,665
Less than 12 Months, Unrealized Loss	(10)	(2)
12 Months or Greater, Fair Value	0	978
12 Months or Greater, Unrealized Loss	0	(1)
Total Fair Value	5,764	2,643
Total Unrealized Loss	(10)	(3)
Mortgage-backed and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,990	4,536
Less than 12 Months, Unrealized Loss	(8)	(13)
12 Months or Greater, Fair Value	273	2,835
12 Months or Greater, Unrealized Loss	(5)	(28)
Total Fair Value	2,263	7,371
Total Unrealized Loss	$ (13)	$ (41)