Fair Value of Financial Assets and Liabilities - Derivative Instruments (Change in Fair Value of Warrant and Derivative Liabilities) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)
Derivatives And Fair Value [Line Items]
Balance, derivative liability	$ 292,000
Changes in estimated fair value	(86,000)
Exercised warrants	(184,000)
Balance, derivative liability	22,000
June 2016 Offering Warrant Liabilities [Member]
Derivatives And Fair Value [Line Items]
Balance, derivative liability	32,000
Changes in estimated fair value	(31,000)
Balance, derivative liability	1,000
November 2016 Offering Warrant Liabilities [Member]
Derivatives And Fair Value [Line Items]
Balance, derivative liability	260,000
Changes in estimated fair value	(55,000)
Exercised warrants	(184,000)
Balance, derivative liability	$ 21,000