Investments (Details 1) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	$ 11,026	$ 10,949
FDIC guaranteed certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	4,993
Corporate notes/bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	1,458
Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	4,575	10,949
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	4,575	10,949
Level 1 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 1 [Member] | Corporate notes/bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 1 [Member] | FDIC guaranteed certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 1 [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	4,575	10,949
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	6,451
Level 2 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	4,993
Level 2 [Member] | Corporate notes/bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current	1,458
Level 2 [Member] | FDIC guaranteed certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 2 [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 3 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 3 [Member] | Corporate notes/bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 3 [Member] | FDIC guaranteed certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sales securities - current
Level 3 [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents