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General - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Jan. 01, 2019
Dec. 31, 2018
May 31, 2018
Debt Instrument [Line Items]          
Cash and cash equivalents $ 44,592,000     $ 7,703,000  
Cash and cash equivalents 44,592,000     $ 7,703,000  
Marketable securities 33,200,000        
Cash, cash equivalents, and marketable securities $ 77,800,000        
Discount rate 12.00%        
Operating lease, payments $ 100,000 $ 100,000      
Payments to be received 3,658,000        
Operating lease, liability 2,847,000        
Revolving Credit Facility          
Debt Instrument [Line Items]          
Long-term debt, gross         $ 68,600,000
Liquidity threshold         20,000,000
Minimum aggregate appraised value plus liquidity threshold         350,000,000
Expense threshold         11,500,000
Cash and cash equivalent threshold         50,000,000
Amended Credit Facility          
Debt Instrument [Line Items]          
Long-term line of credit outstanding 68,600,000        
Liquidity threshold         20,000,000
Minimum aggregate appraised value plus liquidity threshold         350,000,000
Expense threshold         11,500,000.0
Cash and cash equivalent threshold $ 50,000,000       $ 50,000,000
Accounting Standards Update 2016-02          
Debt Instrument [Line Items]          
Right-of-use asset     $ 2,200,000    
Operating lease, liability     2,900,000    
Deferred rent credit     $ 700,000