Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Narrative) (Details)

v3.19.1
Fair Value Measurements (Narrative) (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
May 25, 2018
Dec. 15, 2014
Fair Value Measurements        
Fair value asset transfers between Levels 1, 2 and 3 $ 0      
Convertible Notes due 2023 297,392,000 $ 294,725,000    
Recurring Basis        
Fair Value Measurements        
Asset value   6,346,000    
2019 Convertible Notes        
Fair Value Measurements        
Face amount       $ 172,500,000
Debt instrument, fair value disclosure 192,165,000 174,101,000    
2023 Convertible Notes        
Fair Value Measurements        
Face amount     $ 345,000,000  
Debt instrument, fair value disclosure 397,544,000 339,024,000    
Carrying Value | 2019 Convertible Notes        
Fair Value Measurements        
Convertible Notes due 2023 167,442,000 165,711,000    
Carrying Value | 2023 Convertible Notes        
Fair Value Measurements        
Convertible Notes due 2023 297,392,000 $ 294,725,000    
Other Noncurrent Assets | Recurring Basis        
Fair Value Measurements        
Asset value $ 6,746,000