Description of Business and Summary of Significant Accounting Policies - Cash and Cash Equivalents and Short-term Investments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Level 2 securities:
Gross Unrealized Gains	$ 2
Gross Unrealized Losses	(18)
Less: restricted cash	0	$ (16,000)
Total cash, cash equivalents and short-term investments	67,000
Cost or Amortized Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash	919
Level 2 securities:
Total	67,054
Total Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash	919	43,218
Level 2 securities:
Total	67,038	43,218
Less: restricted cash	0	(16,000)
Total cash, cash equivalents and short-term investments	67,038	27,218
Money market funds | Level 1 | Cost or Amortized Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money market funds	20,525
Money market funds | Level 1 | Total Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money market funds	20,525	0
U.S. government and agency securities | Level 2
Level 2 securities:
Gross Unrealized Gains	1
Gross Unrealized Losses	(5)
U.S. government and agency securities | Level 2 | Cost or Amortized Cost
Level 2 securities:
Cost or Amortized Cost	15,217
U.S. government and agency securities | Level 2 | Total Estimated Fair Value
Level 2 securities:
Total Estimated Fair Value	15,213	0
Corporate debt securities | Level 2
Level 2 securities:
Gross Unrealized Gains	1
Gross Unrealized Losses	(13)
Corporate debt securities | Level 2 | Cost or Amortized Cost
Level 2 securities:
Cost or Amortized Cost	30,393
Corporate debt securities | Level 2 | Total Estimated Fair Value
Level 2 securities:
Total Estimated Fair Value	$ 30,381	$ 0