Fair Value of Financial Assets and Liabilities - Composition of Investments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,321	$ 3,726
Unrealized Gains	3	4
Unrealized (Losses)	(2)	(3)
Aggregate Fair Value	2,322	3,727
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,285	2,260
Unrealized Gains	3	8
Unrealized (Losses)	(3)	(2)
Aggregate Fair Value	2,285	2,266
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	559	515
Unrealized Gains	11	19
Unrealized (Losses)	(9)	(3)
Aggregate Fair Value	561	531
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31
Unrealized Gains	0
Unrealized (Losses)	0
Aggregate Fair Value	31
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	984	860
Unrealized Gains	3	3
Unrealized (Losses)	0	0
Aggregate Fair Value	987	863
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	56	63
Unrealized Gains	0	0
Unrealized (Losses)	(4)	(3)
Aggregate Fair Value	52	60
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,746	2,152
Unrealized Gains	3	6
Unrealized (Losses)	(2)	(3)
Aggregate Fair Value	2,747	2,155
Fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,982	9,576
Unrealized Gains	23	40
Unrealized (Losses)	(20)	(14)
Aggregate Fair Value	8,985	9,602
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	152	72
Unrealized Gains	197	24
Unrealized (Losses)	(9)	(1)
Aggregate Fair Value	340	95
Fixed income and publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,134	9,648
Unrealized Gains	220	64
Unrealized (Losses)	(29)	(15)
Aggregate Fair Value	$ 9,325	$ 9,697