Fair Value Measurements - Fair Value of Financial Assets (Detail) - USD ($) $ in Thousands		Sep. 30, 2015		Dec. 31, 2014		Sep. 30, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		$ 224,835		$ 184,411
Cash and cash equivalents		135,542		146,569		$ 128,629	$ 123,911
Marketable securities		196,697		127,305
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		18,085		11,903
Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		206,750		172,508
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		0		0
Money market funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		18,085		11,903	[1]
Money market funds | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[1]	18,085		11,903
Money market funds | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[1]	0		0
Money market funds | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[1]	0		0
Corporate bonds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	131,630		99,474
Corporate bonds | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Corporate bonds | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	131,630		99,474
Corporate bonds | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Municipal bonds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	17,970		18,137
Municipal bonds | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Municipal bonds | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	17,970		18,137
Municipal bonds | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Commercial paper
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		36,100		9,200
Assets measured at fair value		46,171	[3]	54,399	[4]
Cash and cash equivalents		10,100		45,200
Commercial paper | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		0	[3]	0	[4]
Commercial paper | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		46,171	[3]	54,399	[4]
Commercial paper | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		0	[3]	0	[4]
Certificates of deposit
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	2,492		498
Certificates of deposit | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Certificates of deposit | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	2,492		498
Certificates of deposit | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		$ 0
Agency securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	7,286
Agency securities | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0
Agency securities | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	7,286
Agency securities | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0
Government securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	1,201
Government securities | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0
Government securities | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	1,201
Government securities | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	$ 0

[1] Classified in cash and cash equivalents

[2] Classified in marketable securities

[3] Includes commercial paper with maturities of three months or less at time of purchase of $10.1 million classified in cash and cash equivalents and commercial paper with maturities of greater than three months of $36.1 million classified in marketable securities.

[4] Includes commercial paper with maturities of three months or less at time of purchase of $45.2 million classified in cash and cash equivalents and commercial paper with maturities of greater than three months of $9.2 million classified in marketable securities.