Marketable Securities - Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 69,023	$ 164,845
Gross Unrealized Gain	60	65
Gross Unrealized Loss	(80)	(291)
Fair Value	69,003	164,619
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,224	62,208
Gross Unrealized Gain	11	16
Gross Unrealized Loss	(41)	(134)
Fair Value	31,194	62,090
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,020	49,343
Gross Unrealized Gain	3	5
Gross Unrealized Loss	(13)	(38)
Fair Value	21,010	49,310
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,253	19,280
Gross Unrealized Gain	9	25
Gross Unrealized Loss	(26)	(114)
Fair Value	6,236	19,191
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		18,698
Gross Unrealized Gain		2
Gross Unrealized Loss		0
Fair Value		18,700
Government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,709	11,868
Gross Unrealized Gain	34	14
Gross Unrealized Loss	0	(4)
Fair Value	7,743	11,878
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,817	3,448
Gross Unrealized Gain	3	3
Gross Unrealized Loss	0	(1)
Fair Value	$ 2,820	$ 3,450