Schedule of Investment Securities Classified as Current and Long-Term (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investment [Line Items]
Amortized Cost	$ 9,077	$ 9,072
Gross Unrealized Gains	308	516
Gross Unrealized Losses	(83)	(41)
Fair Value	9,302	9,547
Debt Securities | Tax-exempt municipal securities
Investment [Line Items]
Amortized Cost	2,680	2,931
Gross Unrealized Gains	79	140
Gross Unrealized Losses	(8)	(3)
Fair Value	2,751	3,068
Debt Securities | Residential Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	13	17
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	13	17
Debt Securities | Commercial Mortgage-Backed Securities
Investment [Line Items]
Amortized Cost	1,048	846
Gross Unrealized Gains	11	16
Gross Unrealized Losses	(28)	(19)
Fair Value	1,031	843
Debt Securities | Asset-backed securities
Investment [Line Items]
Amortized Cost	273	28
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(1)	0
Fair Value	273	29
Debt Securities | Corporate debt securities
Investment [Line Items]
Amortized Cost	3,185	3,432
Gross Unrealized Gains	195	299
Gross Unrealized Losses	(41)	(13)
Fair Value	3,339	3,718
Debt Securities | U.S. Treasury and other U.S. government corporations and agencies: | U.S. Treasury and agency obligations
Investment [Line Items]
Amortized Cost	325	365
Gross Unrealized Gains	3	10
Gross Unrealized Losses	0	(1)
Fair Value	328	374
Debt Securities | U.S. Treasury and other U.S. government corporations and agencies: | Mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,553	1,453
Gross Unrealized Gains	19	50
Gross Unrealized Losses	(5)	(5)
Fair Value	$ 1,567	$ 1,498