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Description of Business and Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
shares in Thousands, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Description Of Business And Significant Accounting Policies [Line Items]      
Depreciation $ 1,857 $ 1,962  
Net carrying amount of loaned equipment $ 28,549   $ 28,749
Options, restricted stock and performance based shares excluded from calculation of net loss per share 4,216 4,580  
Share-based compensation expense recognized $ 3,890 $ 3,628  
Generators [Member]      
Description Of Business And Significant Accounting Policies [Line Items]      
Depreciation 827 $ 916  
Carrying Amount of Loaned Equipment [Member]      
Description Of Business And Significant Accounting Policies [Line Items]      
Net carrying amount of loaned equipment $ 4,754   $ 4,656
Short-term Debt [Member]      
Description Of Business And Significant Accounting Policies [Line Items]      
Maturity period of short term investment 1 year