Schedule of Investment Securities Classified as Current and Long-Term (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Amortized Cost	$ 9,690	$ 9,119
Gross Unrealized Gains	404	736
Gross Unrealized Losses	(95)	(8)
Total debt securities, Fair Value	9,999	9,847
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Amortized Cost	2,954	2,890
Gross Unrealized Gains	101	185
Gross Unrealized Losses	(25)	(4)
Total debt securities, Fair Value	3,030	3,071
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	32	33
Gross Unrealized Gains	0	2
Gross Unrealized Losses	0	(1)
Total debt securities, Fair Value	32	34
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	695	615
Gross Unrealized Gains	23	44
Gross Unrealized Losses	(8)	0
Total debt securities, Fair Value	710	659
Debt Securities | Asset-backed Securities
Investment [Line Items]
Amortized Cost	77	66
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(1)	0
Total debt securities, Fair Value	77	68
Debt Securities | Corporate Debt Securities
Investment [Line Items]
Amortized Cost	3,521	3,394
Gross Unrealized Gains	229	402
Gross Unrealized Losses	(30)	(2)
Total debt securities, Fair Value	3,720	3,794
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | U.S. Treasury And Agency Obligations
Investment [Line Items]
Amortized Cost	620	602
Gross Unrealized Gains	8	16
Gross Unrealized Losses	(5)	0
Total debt securities, Fair Value	623	618
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	1,791	1,519
Gross Unrealized Gains	42	85
Gross Unrealized Losses	(26)	(1)
Total debt securities, Fair Value	$ 1,807	$ 1,603