Financial Instruments (Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 3,491	$ 27,724
Less than 12 Months, Unrealized Loss	(8)	(267)
12 Months or Greater, Fair Value	48,041	43,941
12 Months or Greater, Unrealized Loss	(495)	(787)
Total Fair Value	51,532	71,665
Total Unrealized Loss	(503)	(1,054)
Government bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,089	12,019
Less than 12 Months, Unrealized Loss	(1)	(85)
12 Months or Greater, Fair Value	24,177	23,877
12 Months or Greater, Unrealized Loss	(205)	(329)
Total Fair Value	25,266	35,896
Total Unrealized Loss	(206)	(414)
Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,503	10,171
Less than 12 Months, Unrealized Loss	(5)	(107)
12 Months or Greater, Fair Value	13,503	11,545
12 Months or Greater, Unrealized Loss	(104)	(209)
Total Fair Value	15,006	21,716
Total Unrealized Loss	(109)	(316)
Mortgage-backed and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	899	5,534
Less than 12 Months, Unrealized Loss	(2)	(75)
12 Months or Greater, Fair Value	10,361	8,519
12 Months or Greater, Unrealized Loss	(186)	(249)
Total Fair Value	11,260	14,053
Total Unrealized Loss	$ (188)	$ (324)