Financial Instruments (Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 48,601	$ 34,105
Less than 12 months, unrealized loss	(614)	(315)
12 months or greater, fair value	471	1,968
12 months or greater, unrealized loss	(17)	(106)
Fair Value	49,072	36,073
Unrealized Loss	(631)	(421)
U.S. government notes
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	26,411	13,757
Less than 12 months, unrealized loss	(215)	(48)
12 months or greater, fair value	0	0
12 months or greater, unrealized loss	0	0
Fair Value	26,411	13,757
Unrealized Loss	(215)	(48)
U.S. government agencies
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	1,014	864
Less than 12 months, unrealized loss	(11)	(3)
12 months or greater, fair value	0	0
12 months or greater, unrealized loss	0	0
Fair Value	1,014	864
Unrealized Loss	(11)	(3)
Foreign government bonds
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	956	885
Less than 12 months, unrealized loss	(7)	(18)
12 months or greater, fair value	0	36
12 months or greater, unrealized loss	0	(5)
Fair Value	956	921
Unrealized Loss	(7)	(23)
Municipal securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	3,461	1,116
Less than 12 months, unrealized loss	(63)	(3)
12 months or greater, fair value	46	41
12 months or greater, unrealized loss	(2)	(1)
Fair Value	3,507	1,157
Unrealized Loss	(65)	(4)
Corporate debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	6,184	9,192
Less than 12 months, unrealized loss	(111)	(202)
12 months or greater, fair value	166	784
12 months or greater, unrealized loss	(5)	(76)
Fair Value	6,350	9,976
Unrealized Loss	(116)	(278)
Agency mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	10,184	5,783
Less than 12 months, unrealized loss	(206)	(34)
12 months or greater, fair value	259	721
12 months or greater, unrealized loss	(10)	(23)
Fair Value	10,443	6,504
Unrealized Loss	(216)	(57)
Asset-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	391	2,508
Less than 12 months, unrealized loss	(1)	(7)
12 months or greater, fair value	0	386
12 months or greater, unrealized loss	0	(1)
Fair Value	391	2,894
Unrealized Loss	$ (1)	$ (8)