Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Carrying Amount	$ 59,433	$ 101,537
Gross Unrealized Gain	1	0
Gross Unrealized Loss	(4)	(72)
Fair Value	59,430	101,465
Cash and Cash Equivalents	35,045	29,117
Short-Term Marketable Securities	24,385	65,855
Long-Term Marketable Securities	0	6,493
Cash
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Carrying Amount	13,984	17,889
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	13,984	17,889
Cash and Cash Equivalents	13,984	17,889
Short-Term Marketable Securities	0	0
Long-Term Marketable Securities	0	0
Money market funds | Level I
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Carrying Amount	21,061	11,228
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	21,061	11,228
Cash and Cash Equivalents	21,061	11,228
Short-Term Marketable Securities	0	0
Long-Term Marketable Securities	0	0
U.S. government and agency securities | Level II
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Carrying Amount	24,388	72,420
Gross Unrealized Gain	1	0
Gross Unrealized Loss	(4)	(72)
Fair Value	24,385	72,348
Cash and Cash Equivalents	0	0
Short-Term Marketable Securities	24,385	65,855
Long-Term Marketable Securities	$ 0	$ 6,493