Fair Value Measurements (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Financial Assets:
Held-to-maturity Securities, Fair Value		$ 2,448
Fair Value Measurements, Recurring [Member]
Financial Assets:
Total		2,469	$ 5,026
Financial Liabilities:
Total
Fair Value Measurements, Recurring [Member] | Corporate Fixed Income Debt Securities [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value		2,448	5,023
Fair Value Measurements, Recurring [Member] | Money Market Funds [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value		21	3
Financial Liabilities:
Common stock warrant liability	[1]
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 1 [Member]
Financial Assets:
Total		21	3
Financial Liabilities:
Total
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 1 [Member] | Corporate Fixed Income Debt Securities [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 1 [Member] | Money Market Funds [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value		21	3
Financial Liabilities:
Common stock warrant liability	[1]
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 2 [Member]
Financial Assets:
Total		2,448	5,023
Financial Liabilities:
Total
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 2 [Member] | Corporate Fixed Income Debt Securities [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value		2,448	5,023
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 2 [Member] | Money Market Funds [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value
Financial Liabilities:
Common stock warrant liability	[1]
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 3 [Member]
Financial Assets:
Total
Financial Liabilities:
Total
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 3 [Member] | Corporate Fixed Income Debt Securities [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value
Fair Value Measurements, Recurring [Member] | Fair Value Inputs, Level 3 [Member] | Money Market Funds [Member]
Financial Assets:
Held-to-maturity Securities, Fair Value
Financial Liabilities:
Common stock warrant liability	[1]

[1] The change in the fair value of the common stock warrant and convertible notes payable for the three and nine months ended September 30, 2018 was recorded as a decrease to other income (expense) and interest expense of $1 and $1, respectively, in the statements of operations and comprehensive loss.