Fair Value of Financial Assets and Liabilities - Derivative Instruments (Change in Fair Value of Warrant and Derivative Liabilities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivatives And Fair Value [Line Items]
Balance, derivative liability	$ 2,443,000
Changes in estimated fair value	(2,010,000)
Exercised warrants	(119,000)
Settlement of liabilities	(22,000)
Balance, derivative liability	292,000	$ 2,443,000
Dilutive Financing Derivative Liability [Member]
Derivatives And Fair Value [Line Items]
Balance, derivative liability	126,000
Changes in estimated fair value	(104,000)	(611,000)
Settlement of liabilities	(22,000)
Balance, derivative liability		126,000
November 2016 Warrant Derivative Liability [Member]
Derivatives And Fair Value [Line Items]
Balance, derivative liability	2,043,000
Changes in estimated fair value	(1,664,000)
Exercised warrants	(119,000)
Balance, warrant liability	260,000
Balance, derivative liability	260,000	2,043,000
June 2016 Offering Warrants [Member]
Derivatives And Fair Value [Line Items]
Balance, warrant liability	274,000
Balance, derivative liability	274,000
Changes in estimated fair value	(242,000)
Balance, warrant liability	32,000	274,000
Balance, derivative liability	$ 32,000	$ 274,000