Financial Instruments (Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 14,312	$ 14,300
Less than 12 Months, Unrealized Loss	(103)	(326)
12 Months or Greater, Fair Value	3,703	719
12 Months or Greater, Unrealized Loss	(108)	(32)
Total Fair Value	18,015	15,019
Total Unrealized Loss	(211)	(358)
US government notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	3,623	4,404
Less than 12 Months, Unrealized Loss	(4)	(37)
12 Months or Greater, Fair Value	296	0
12 Months or Greater, Unrealized Loss	(4)	0
Total Fair Value	3,919	4,404
Total Unrealized Loss	(8)	(37)
U.S. government agencies
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,457	496
Less than 12 Months, Unrealized Loss	(4)	(3)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Loss	0	0
Total Fair Value	1,457	496
Total Unrealized Loss	(4)	(3)
Foreign government bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	480	899
Less than 12 Months, Unrealized Loss	(6)	(23)
12 Months or Greater, Fair Value	208	83
12 Months or Greater, Unrealized Loss	(5)	(3)
Total Fair Value	688	982
Total Unrealized Loss	(11)	(26)
Municipal securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	268	1,210
Less than 12 Months, Unrealized Loss	(1)	(32)
12 Months or Greater, Fair Value	198	99
12 Months or Greater, Unrealized Loss	(3)	(4)
Total Fair Value	466	1,309
Total Unrealized Loss	(4)	(36)
Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	4,901	2,583
Less than 12 Months, Unrealized Loss	(62)	(62)
12 Months or Greater, Fair Value	294	69
12 Months or Greater, Unrealized Loss	(12)	(5)
Total Fair Value	5,195	2,652
Total Unrealized Loss	(74)	(67)
Agency residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,763	4,065
Less than 12 Months, Unrealized Loss	(5)	(167)
12 Months or Greater, Fair Value	2,518	468
12 Months or Greater, Unrealized Loss	(83)	(20)
Total Fair Value	4,281	4,533
Total Unrealized Loss	(88)	(187)
Asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,454	643
Less than 12 Months, Unrealized Loss	(2)	(2)
12 Months or Greater, Fair Value	189	0
12 Months or Greater, Unrealized Loss	(1)	0
Total Fair Value	1,643	643
Total Unrealized Loss	(3)	(2)
Fixed-income Bond Funds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	366
Less than 12 Months, Unrealized Loss	(19)
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Loss	0
Total Fair Value	366
Total Unrealized Loss	$ (19)