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Convertible Senior Notes - Additional Information (Details)
1 Months Ended 9 Months Ended 12 Months Ended
Nov. 30, 2017
USD ($)
$ / shares
shares
Sep. 30, 2018
USD ($)
d
$ / shares
shares
Sep. 30, 2017
shares
Dec. 31, 2017
USD ($)
Sep. 28, 2018
$ / shares
Debt Instrument [Line Items]          
Number of shares of common stock from capped calls cover, subject to anti-dilution adjustments | shares   5,770,318 3,041,553    
Convertible Senior Notes          
Debt Instrument [Line Items]          
Aggregate principal amount | $ $ 115,000,000 $ 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 2966.26%        
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      
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  
Number of shares of common stock from capped calls cover, subject to anti-dilution adjustments | shares   2,436,570      
Share price | $ / shares         $ 57.64
Convertible Senior Notes | 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%      
Convertible Senior Notes | 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