Basis of Presentation and Summary of Significant Accounting Policies (Effect of New Accounting Principle) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue (contract liabilities), non-current		$ 77
Accumulated deficit	$ (373,887)	(344,014)	$ (345,858)
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue (contract liabilities), non-current			1,921
Accumulated deficit			$ (345,858)
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue (contract liabilities), non-current		(1,844)
Accumulated deficit		$ 1,844