Investments (Details 1) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	$ 17,177	$ 11,026
Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current		1,458
US Treasury Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	14,356
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current		4,993
Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	2,821	4,575
Fair Value, Inputs, Level 1 [Member] | Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	14,356	6,451
Fair Value, Inputs, Level 2 [Member] | Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current		1,458
Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	14,356
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current		4,993
Significant unobservable inputs (Level 3 )
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Significant unobservable inputs (Level 3 ) | Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Significant unobservable inputs (Level 3 ) | US Treasury Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Significant unobservable inputs (Level 3 ) | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	2,821	4,575
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	2,821	4,575
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents
Money Market Funds [Member] | Significant unobservable inputs (Level 3 )
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents