Financial Instruments (Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 24,525	$ 12,953
Less than 12 Months, Unrealized Loss	(367)	(35)
12 Months or Greater, Fair Value	518	8,403
12 Months or Greater, Unrealized Loss	(7)	(39)
Total Fair Value	25,043	21,356
Total Unrealized Loss	(374)	(74)
Government bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	4,168	6,752
Less than 12 Months, Unrealized Loss	(50)	(20)
12 Months or Greater, Fair Value	79	4,590
12 Months or Greater, Unrealized Loss	0	(10)
Total Fair Value	4,247	11,342
Total Unrealized Loss	(50)	(30)
Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	15,561	1,665
Less than 12 Months, Unrealized Loss	(232)	(2)
12 Months or Greater, Fair Value	113	978
12 Months or Greater, Unrealized Loss	(1)	(1)
Total Fair Value	15,674	2,643
Total Unrealized Loss	(233)	(3)
Mortgage-backed and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	4,796	4,536
Less than 12 Months, Unrealized Loss	(85)	(13)
12 Months or Greater, Fair Value	326	2,835
12 Months or Greater, Unrealized Loss	(6)	(28)
Total Fair Value	5,122	7,371
Total Unrealized Loss	$ (91)	$ (41)