Schedule of Investment Securities Classified as Current and Long-Term (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 9,583	$ 9,550
Gross Unrealized Gains	503	377
Gross Unrealized Losses	(51)	(127)
Total debt securities, Fair Value	10,035	9,800
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Amortized Cost	3,006	2,911
Gross Unrealized Gains	148	93
Gross Unrealized Losses	(3)	(33)
Total debt securities, Fair Value	3,151	2,971
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	18	22
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total debt securities, Fair Value	18	22
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	698	662
Gross Unrealized Gains	15	20
Gross Unrealized Losses	(17)	(9)
Total debt securities, Fair Value	696	673
Debt Securities | Asset-backed Securities
Investment [Line Items]
Amortized Cost	40	63
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	(1)
Total debt securities, Fair Value	41	63
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Amortized Cost	3,431	3,474
Gross Unrealized Gains	289	223
Gross Unrealized Losses	(12)	(30)
Total debt securities, Fair Value	3,708	3,667
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | U.S. Treasury And Agency Obligations
Investment [Line Items]
Amortized Cost	846	584
Gross Unrealized Gains	8	6
Gross Unrealized Losses	(2)	(6)
Total debt securities, Fair Value	852	584
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	1,544	1,834
Gross Unrealized Gains	42	34
Gross Unrealized Losses	(17)	(48)
Total debt securities, Fair Value	$ 1,569	$ 1,820