Investments (Details 1) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments:
Total	$ 17,084	$ 11,026
Fair value on a recurring basis [Member] | Corporate notes/bonds [Member]
Investments:
Available-for-sale securities - current:		1,458
Fair value on a recurring basis [Member] | U.S. Treasury securities [Member]
Investments:
Available-for-sale securities - current:	10,135
Fair value on a recurring basis [Member] | Commercial paper [Member]
Investments:
Available-for-sale securities - current:	4,444	4,993
Level 1 [Member]
Investments:
Total	2,505	4,575
Level 1 [Member] | Fair value on a recurring basis [Member] | Corporate notes/bonds [Member]
Investments:
Available-for-sale securities - current:
Level 1 [Member] | Fair value on a recurring basis [Member] | U.S. Treasury securities [Member]
Investments:
Available-for-sale securities - current:
Level 1 [Member] | Fair value on a recurring basis [Member] | Commercial paper [Member]
Investments:
Available-for-sale securities - current:
Level 2 [Member]
Investments:
Total	14,579	6,451
Level 2 [Member] | Fair value on a recurring basis [Member] | Corporate notes/bonds [Member]
Investments:
Available-for-sale securities - current:		1,458
Level 2 [Member] | Fair value on a recurring basis [Member] | U.S. Treasury securities [Member]
Investments:
Available-for-sale securities - current:	10,135
Level 2 [Member] | Fair value on a recurring basis [Member] | Commercial paper [Member]
Investments:
Available-for-sale securities - current:	4,444	4,993
Level 3 [Member]
Investments:
Total
Level 3 [Member] | Fair value on a recurring basis [Member] | Corporate notes/bonds [Member]
Investments:
Available-for-sale securities - current:
Level 3 [Member] | Fair value on a recurring basis [Member] | U.S. Treasury securities [Member]
Investments:
Available-for-sale securities - current:
Level 3 [Member] | Fair value on a recurring basis [Member] | Commercial paper [Member]
Investments:
Available-for-sale securities - current:
Money market funds [Member] | Fair value on a recurring basis [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents:	2,505	4,575
Money market funds [Member] | Level 1 [Member] | Fair value on a recurring basis [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents:	2,505	4,575
Money market funds [Member] | Level 2 [Member] | Fair value on a recurring basis [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents:
Money market funds [Member] | Level 3 [Member] | Fair value on a recurring basis [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents: