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Convertible Senior Notes (Details)
$ / shares in Units, $ in Thousands
9 Months Ended
Jul. 23, 2018
USD ($)
$ / shares
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Debt Instrument [Line Items]      
Proceeds from issuance of convertible senior notes   $ 138,953 $ 0
Proceeds from issuance of convertible debt, net of underwriting discounts $ 138,953    
Debt issuance costs, gross 4,797    
Debt issuance costs, gross, carrying amount of liability component 3,832    
Debt issuance costs, gross, carrying amount of equity component 965    
1.75% Convertible Senior Notes due 2023      
Debt Instrument [Line Items]      
Proceeds from issuance of convertible senior notes 143,750    
Proceeds from issuance of convertible debt, net of underwriting discounts $ 138,953    
Stated interest rate 1.75%    
Debt instrument, redemption price, percentage 100.00%    
Debt instrument, convertible, conversion ratio 0.0327332    
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares $ 30.55    
Premium percentage of conversion price 30.00%    
Debt issuance costs, gross, carrying amount of equity component   965  
Public Offerings      
Debt Instrument [Line Items]      
Offering price (in dollars per share) | $ / shares $ 23.50    
Fair Value, Inputs, Level 2 | 1.75% Convertible Senior Notes due 2023      
Debt Instrument [Line Items]      
Debt instrument, fair value   $ 129,195