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Convertible Senior Notes - Additional Information (Details) - Convertible Senior Notes
1 Months Ended 6 Months Ended 12 Months Ended
Nov. 30, 2017
USD ($)
$ / shares
shares
Jun. 30, 2019
USD ($)
d
$ / shares
Jun. 30, 2018
d
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Jun. 28, 2019
$ / shares
Debt Instrument [Line Items]            
Aggregate principal amount $ 115,000,000.0 $ 115,000,000   $ 115,000,000    
Debt instrument, interest rate 1.50%          
Debt instrument, maturity date Nov. 01, 2022          
Debt instrument, interest rate terms   Interest is payable semiannually in arrears on May 1 and November 1 of each year, commencing on May 1, 2018.        
Interest payment commencing date May 01, 2018          
Debt Conversion, initial conversion rate | shares 29.6626          
Principal amount per note used in conversion rate $ 1,000          
Conversion price per share | $ / shares $ 33.71          
Shares issuable upon conversion of debt | shares 3,400,000          
Description of convertible notes at option of holders   Prior to the close of business on the business day immediately preceding May 1, 2022, the Notes will be convertible at the option of holders during certain periods, only upon satisfaction of certain conditions set forth below. On or after May 1, 2022, until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert all or any portion of their Notes at the conversion rate at any time regardless of whether the conditions set forth below have been met.        
Number of trading days | d     30      
Implied interest rate   6.93%        
Debt instrument, fair value   $ 92,100,000        
Difference between aggregate principal amount and estimated fair value of liability component recoded in additional paid-in capital   $ 22,900,000        
Initial strike price of capped call options | $ / shares   $ 33.71        
Capped call options, cap price | $ / shares   $ 47.20        
Cost of purchased capped call options       $ 12,900,000 $ 12,900,000  
Share price | $ / shares           $ 89.42
Scenario One            
Debt Instrument [Line Items]            
Number of trading days | d   20        
Number of consecutive trading days | d   30        
Percentage of last reported sale price to conversion price on each applicable trading day   130.00%        
Scenario Two            
Debt Instrument [Line Items]            
Number of consecutive trading days | d   5        
Percentage of last reported sale price to conversion price on each applicable trading day   98.00%        
Notes measurement period   5 days