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Investments and Fair Value Measurements - Summary of Amortized Cost and Fair Value of the Available-For-Sale Investment Securities Portfolio (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost $ 12,842 $ 52,613
Unrealized Gains   0
Unrealized Losses (1) (19)
Estimated Fair Value 12,841 52,594
Cash Equivalents [Member] | Money Market Funds [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 8,943 1,549
Unrealized Gains   0
Unrealized Losses   0
Estimated Fair Value 8,943 1,549
Short-term Investments [Member] | Commercial Paper [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 1,399 21,248
Unrealized Gains   0
Unrealized Losses   0
Estimated Fair Value 1,399 21,248
Short-term Investments [Member] | U.S. Government Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 2,500 19,267
Unrealized Gains   0
Unrealized Losses (1) (9)
Estimated Fair Value $ 2,499 19,258
Short-term Investments [Member] | Certificates of Deposit [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost   3,049
Unrealized Gains   0
Unrealized Losses   (2)
Estimated Fair Value   3,047
Short-term Investments [Member] | U.S. Government Agency Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost   5,000
Unrealized Gains   0
Unrealized Losses   (6)
Estimated Fair Value   4,994
Long-term investments [Member] | U.S. Government Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost   2,500
Unrealized Gains   0
Unrealized Losses   (2)
Estimated Fair Value   $ 2,498