Summary of Marketable Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,469	$ 587
Unrealized Gains	2	10
Unrealized Losses	(4)	0
Total Estimated Fair Value	1,467	597
U.S. government and agency debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	342	236
Unrealized Gains	1	2
Unrealized Losses	(2)	0
Total Estimated Fair Value	341	238
Mortgage and asset-backed debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	70	171
Unrealized Gains	0	3
Unrealized Losses	(1)	0
Total Estimated Fair Value	69	174
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	713	158
Unrealized Gains	1	5
Unrealized Losses	(1)	0
Total Estimated Fair Value	713	163
U.S. state and local municipal debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	19	15
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Estimated Fair Value	19	15
Other debt and equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost	325	7
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Estimated Fair Value	$ 325	$ 7