Financial Instruments (Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 12,176	$ 16,337
Less than 12 Months, Unrealized Loss	(536)	(234)
12 Months or Greater, Fair Value	1,956	2,784
12 Months or Greater, Unrealized Loss	(158)	(58)
Total Fair Value	14,132	19,121
Total Unrealized Loss	(694)	(292)
U.S. government notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value		4,490
Less than 12 Months, Unrealized Loss		(4)
12 Months or Greater, Fair Value		0
12 Months or Greater, Unrealized Loss		0
Total Fair Value		4,490
Total Unrealized Loss		(4)
U.S. government agencies
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value		830
Less than 12 Months, Unrealized Loss		(1)
12 Months or Greater, Fair Value		0
12 Months or Greater, Unrealized Loss		0
Total Fair Value		830
Total Unrealized Loss		(1)
Foreign government bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	873	255
Less than 12 Months, Unrealized Loss	(19)	(7)
12 Months or Greater, Fair Value	27	43
12 Months or Greater, Unrealized Loss	(4)	(3)
Total Fair Value	900	298
Total Unrealized Loss	(23)	(10)
Municipal securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	356	877
Less than 12 Months, Unrealized Loss	(3)	(3)
12 Months or Greater, Fair Value	23	174
12 Months or Greater, Unrealized Loss	(1)	(3)
Total Fair Value	379	1,051
Total Unrealized Loss	(4)	(6)
Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	7,152	5,851
Less than 12 Months, Unrealized Loss	(187)	(112)
12 Months or Greater, Fair Value	651	225
12 Months or Greater, Unrealized Loss	(52)	(10)
Total Fair Value	7,803	6,076
Total Unrealized Loss	(239)	(122)
Agency mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,355	609
Less than 12 Months, Unrealized Loss	(8)	(1)
12 Months or Greater, Fair Value	739	2,168
12 Months or Greater, Unrealized Loss	(16)	(41)
Total Fair Value	2,094	2,777
Total Unrealized Loss	(24)	(42)
Asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,650	2,388
Less than 12 Months, Unrealized Loss	(4)	(4)
12 Months or Greater, Fair Value	230	174
12 Months or Greater, Unrealized Loss	(1)	(1)
Total Fair Value	1,880	2,562
Total Unrealized Loss	(5)	(5)
Fixed-income bond funds(3)
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	0	347
Less than 12 Months, Unrealized Loss	0	(38)
12 Months or Greater, Fair Value	286	0
12 Months or Greater, Unrealized Loss	(84)	0
Total Fair Value	286	347
Total Unrealized Loss	(84)	(38)
Marketable equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	790	690
Less than 12 Months, Unrealized Loss	(315)	(64)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Loss	0	0
Total Fair Value	790	690
Total Unrealized Loss	$ (315)	$ (64)