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Fair Value Measurements - Summary of Investments and Cash Equivalents (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Carrying Amount $ 40,419 $ 63,137
Gross Unrealized Gain 1 0
Gross Unrealized Loss 0 (1)
Fair Value 40,420 63,136
Cash and Cash Equivalents 39,424 58,056
Short-Term Marketable Securities 996 5,080
Cash    
Schedule of Available-for-sale Securities [Line Items]    
Carrying Amount 11,826 20,129
Gross Unrealized Gain 0 0
Gross Unrealized Loss 0 0
Fair Value 11,826 20,129
Cash and Cash Equivalents 11,826 20,129
Short-Term Marketable Securities 0 0
Level I:    
Schedule of Available-for-sale Securities [Line Items]    
Carrying Amount 6,191 16,639
Gross Unrealized Gain 0 0
Gross Unrealized Loss 0 0
Fair Value 6,191 16,639
Cash and Cash Equivalents 5,195 16,142
Short-Term Marketable Securities 996 497
Level I: | Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Carrying Amount 5,195 16,142
Gross Unrealized Gain 0 0
Gross Unrealized Loss 0 0
Fair Value 5,195 16,142
Cash and Cash Equivalents 5,195 16,142
Short-Term Marketable Securities 0 0
Level I: | U.S. treasury securities    
Schedule of Available-for-sale Securities [Line Items]    
Carrying Amount 996 497
Gross Unrealized Gain 0
Gross Unrealized Loss 0
Fair Value 996 497
Cash and Cash Equivalents 0
Short-Term Marketable Securities 996 497
Level II: | U.S. government and agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Carrying Amount 22,402 26,369
Gross Unrealized Gain 1 0
Gross Unrealized Loss 0 (1)
Fair Value 22,403 26,368
Cash and Cash Equivalents 22,403 21,785
Short-Term Marketable Securities $ 0 $ 4,583