Fair Value Measurements - Fair Value of Financial Assets (Details) - USD ($) $ in Thousands		Jun. 30, 2016		Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		$ 135,561		$ 224,645
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		9,296		18,114
Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		126,265		206,531
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value		0		0
Money market funds (1)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[1]	9,296		18,114
Money market funds (1) | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[1]	9,296		18,114
Money market funds (1) | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[1]	0		0
Money market funds (1) | 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]	87,606		124,829
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]	87,606		124,829
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]	9,962		16,223
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]	9,962		16,223
Municipal bonds | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Government securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	9,713		8,688
Government securities | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Government securities | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	9,713		8,688
Government securities | 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				10,100
Assets measured at fair value		10,472	[2]	47,294	[3]
Commercial paper with maturities of three months or less				37,200
Commercial paper | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		0	[2]	0	[3]
Commercial paper | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		10,472	[2]	47,294	[3]
Commercial paper | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value		0	[2]	0	[3]
Agency securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	7,264		7,253
Agency securities | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Agency securities | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	7,264		7,253
Agency securities | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	0		0
Certificates of deposit
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	1,248		2,244
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]	1,248		2,244
Certificates of deposit | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured at fair value	[2]	$ 0		$ 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 $37.2 million classified in marketable securities.