Summary of Business and Significant Accounting Policies, Liquidity (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018		Dec. 31, 2017
Liquidity [Abstract]
Net loss	$ (4,003)	$ (8,276)	$ (8,609)	$ (17,388)	$ (41,244)
Accumulated deficit	(273,322)		(273,322)		(264,713)	[1]
Cash, cash equivalents and restricted cash	10,857	$ 17,220	10,857	$ 17,220	$ 11,503		$ 28,837
Equity Distribution Agreement [Abstract]
Common stock under agreement for net proceeds	$ 1,504		$ 1,504
Common stock under agreement for net proceeds(in shares)	570		570
Maximum [Member]
Equity Distribution Agreement [Abstract]
Aggregate offering price	$ 50,000		$ 50,000
JMP Securities [Member] | Maximum [Member]
Equity Distribution Agreement [Abstract]
Aggregate offering price	$ 13,000		$ 13,000

[1] Derived from the Company's audited consolidated financial statements as of December 31, 2018.