Financial Instruments (Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 55,070	$ 48,601
Less than 12 Months, Unrealized Loss	(238)	(614)
12 Months or Greater, Fair Value	7,485	471
12 Months or Greater, Unrealized Loss	(76)	(17)
Total Fair Value	62,555	49,072
Total Unrealized Loss	(314)	(631)
U.S. government notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	26,626	26,411
Less than 12 Months, Unrealized Loss	(115)	(215)
12 Months or Greater, Fair Value	5,409	0
12 Months or Greater, Unrealized Loss	(48)	0
Total Fair Value	32,035	26,411
Total Unrealized Loss	(163)	(215)
U.S. government agencies
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	2,751	1,014
Less than 12 Months, Unrealized Loss	(9)	(11)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Loss	0	0
Total Fair Value	2,751	1,014
Total Unrealized Loss	(9)	(11)
Foreign government bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,362	956
Less than 12 Months, Unrealized Loss	(13)	(7)
12 Months or Greater, Fair Value	123	0
12 Months or Greater, Unrealized Loss	(2)	0
Total Fair Value	1,485	956
Total Unrealized Loss	(15)	(7)
Municipal securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	2,118	3,461
Less than 12 Months, Unrealized Loss	(6)	(63)
12 Months or Greater, Fair Value	432	46
12 Months or Greater, Unrealized Loss	(4)	(2)
Total Fair Value	2,550	3,507
Total Unrealized Loss	(10)	(65)
Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	11,153	6,184
Less than 12 Months, Unrealized Loss	(27)	(111)
12 Months or Greater, Fair Value	793	166
12 Months or Greater, Unrealized Loss	(8)	(5)
Total Fair Value	11,946	6,350
Total Unrealized Loss	(35)	(116)
Agency mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	8,450	10,184
Less than 12 Months, Unrealized Loss	(62)	(206)
12 Months or Greater, Fair Value	728	259
12 Months or Greater, Unrealized Loss	(14)	(10)
Total Fair Value	9,178	10,443
Total Unrealized Loss	(76)	(216)
Asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	2,572	391
Less than 12 Months, Unrealized Loss	(4)	(1)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Loss	0	0
Total Fair Value	2,572	391
Total Unrealized Loss	(4)	$ (1)
Marketable equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	38
Less than 12 Months, Unrealized Loss	(2)
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Loss	0
Total Fair Value	38
Total Unrealized Loss	$ (2)