Marketable Securities - Classified as Available-for-Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 189,997	$ 112,458
Gross Unrealized Gain	91	131
Gross Unrealized Loss	(212)	(45)
Fair Value	189,876	112,544
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,856	45,414
Gross Unrealized Gain	33	75
Gross Unrealized Loss	(90)	(7)
Fair Value	69,799	45,482
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	56,285	21,222
Gross Unrealized Gain	13	12
Gross Unrealized Loss	(36)	(8)
Fair Value	56,262	21,226
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,596	13,890
Gross Unrealized Gain	2	2
Gross Unrealized Loss	0	0
Fair Value	30,598	13,892
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,348	15,110
Gross Unrealized Gain	31	33
Gross Unrealized Loss	(81)	(26)
Fair Value	21,298	15,117
Government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,208	12,255
Gross Unrealized Gain	7	9
Gross Unrealized Loss	(5)	0
Fair Value	9,210	12,264
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,704	3,817
Gross Unrealized Gain	5	0
Gross Unrealized Loss	0	(4)
Fair Value	2,709	3,813
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		750
Gross Unrealized Gain		0
Gross Unrealized Loss		0
Fair Value		$ 750