Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ (375,109)	$ (174,311)
Gross Unrealized Losses	(381)	(113)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value	(81,467)	(36,753)
Gross Unrealized Losses	(21)	(15)
Corporate notes and bonds securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	(293,642)	(137,558)
Gross Unrealized Losses	$ (360)	$ (98)