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Fair value disclosures - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
Mar. 31, 2020
Dec. 31, 2019
Fair Value, Measurements, Recurring | Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets $ 7,662,000 $ 7,638,000
Fair Value, Measurements, Recurring | Money market funds | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets 7,662,000 7,638,000
Fair Value, Measurements, Recurring | Money market funds | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets 0 0
Fair Value, Measurements, Recurring | Money market funds | Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets 0 0
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets 528,000 616,000
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets 528,000 616,000
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets 0 0
Fair Value, Measurements, Recurring | Mutual funds held in Rabbi Trust | Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets 0 0
Fair Value, Measurements, Nonrecurring    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value, assets $ 0 $ 0