Schedule of Investment Securities Classified as Current and Long-Term (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 9,673	$ 9,550
Gross Unrealized Gains	456	377
Gross Unrealized Losses	(81)	(127)
Total debt securities, Fair Value	10,048	9,800
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Amortized Cost	3,067	2,911
Gross Unrealized Gains	116	93
Gross Unrealized Losses	(17)	(33)
Total debt securities, Fair Value	3,166	2,971
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	21	22
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Total debt securities, Fair Value	20	22
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	656	662
Gross Unrealized Gains	20	20
Gross Unrealized Losses	(11)	(9)
Total debt securities, Fair Value	665	673
Debt Securities | Asset-backed Securities
Investment [Line Items]
Amortized Cost	49	63
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	(1)
Total debt securities, Fair Value	50	63
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Amortized Cost	3,493	3,474
Gross Unrealized Gains	275	223
Gross Unrealized Losses	(13)	(30)
Total debt securities, Fair Value	3,755	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	517	584
Gross Unrealized Gains	7	6
Gross Unrealized Losses	(4)	(6)
Total debt securities, Fair Value	520	584
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	1,870	1,834
Gross Unrealized Gains	37	34
Gross Unrealized Losses	(35)	(48)
Total debt securities, Fair Value	$ 1,872	$ 1,820