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Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Amortized Cost    
Amortized cost $ 70,121 $ 99,171
Gross Unrealized Gains    
Gross Unrealized Gains 1 4
Gross Unrealized Losses    
Gross Unrealized Losses (7) (36)
Fair Value    
Fair Value 70,115 99,139
Other-than-temporarily impairment 0 0
Accrued interest 200 300
U.S. government and municipal obligations    
Amortized Cost    
Maturing in one year or less 19,362 26,164
Gross Unrealized Gains    
Maturing in one year or less 1 3
Gross Unrealized Losses    
Maturing in one year or less   (9)
Fair Value    
Maturing in one year or less 19,363 26,158
Corporate debt securities    
Amortized Cost    
Maturing in one year or less 50,759 73,007
Gross Unrealized Gains    
Maturing in one year or less   1
Gross Unrealized Losses    
Maturing in one year or less (7) (27)
Fair Value    
Maturing in one year or less $ 50,752 $ 72,981