Available-For-Sale Investments in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	$ 109,039	$ 39,721
Less Than 12 Months, Unrealized Losses	(204)	(15)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	109,039	39,721
Total Unrealized Losses	(204)	(15)
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	9,296	9,802
Less Than 12 Months, Unrealized Losses	(15)	(7)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	9,296	9,802
Total Unrealized Losses	(15)	(7)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	11,969
Less Than 12 Months, Unrealized Losses	(5)
12 Months or Greater, Estimated Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total Estimated Fair Value	11,969
Total Unrealized Losses	(5)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	82,745	29,919
Less Than 12 Months, Unrealized Losses	(182)	(8)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	82,745	29,919
Total Unrealized Losses	(182)	(8)
Securities of government-sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	5,029
Less Than 12 Months, Unrealized Losses	(2)
12 Months or Greater, Estimated Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total Estimated Fair Value	5,029
Total Unrealized Losses	$ (2)