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Investments and Fair Value Measurements - Narrative (Details) - USD ($)
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Fair Value Disclosures [Abstract]    
Proceeds from sale of convertible note held for investment $ 1,923,000 $ 0
Carrying value of convertible notes sold 1,600,000  
Total proceeds from sale of convertible notes receivable 2,100,000  
Level 2 | Convertible debt    
Debt Instrument [Line Items]    
Fair value of long-term debt 269,700,000  
Nonrecurring    
Debt Instrument [Line Items]    
Assets measured at fair value on nonrecurring basis 0  
Liabilities measured at fair value on nonrecurring basis $ 0