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Long-Term Debt - 2024 and 2020 Senior Convertible Notes, and Related Note Hedge and Warrant Transactions (Details)
$ / shares in Units, shares in Millions
 3 Months Ended 9 Months Ended
Jun. 03, 2019
USD ($)
day
$ / shares
May 31, 2019
USD ($)
$ / shares
shares
May 31, 2019
USD ($)
$ / shares
shares
Sep. 30, 2019
USD ($)
$ / shares
Jun. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
day
$ / shares
shares
Sep. 30, 2018
USD ($)
May 29, 2019
USD ($)
$ / shares
Dec. 31, 2018
USD ($)
Mar. 11, 2015
USD ($)
Mar. 09, 2015
shares
Debt Instrument [Line Items]                        
Convertible note hedge   $ 72,000,000.0 $ 72,000,000.0                  
Initial strike price (in usd per share) | $ / shares   $ 109.43 $ 109.43                  
Aggregate proceeds warrant amount     $ 47,600,000       $ 47,600,000 $ 0        
Call spread transactions   $ 24,400,000 24,400,000                  
Use of proceeds repayment of long term debt   232,700,000 $ 232,700,000   $ 232,700,000              
Payments on long-term debt   $ 221,100,000     221,100,000   $ 221,091,000 0        
Use of proceeds repurchases of common stock                 $ 19,600,000      
Closing price (in usd per share) | $ / shares                 $ 62.53      
Unwind transactions (in shares) | shares             1.3          
Proceeds from hedge unwind         9,000,000.0   $ 9,038,000 0        
Payment for warrant unwind         4,200,000   4,184,000 0        
Proceeds for combined unwind transactions, net         4,900,000              
Loss on extinguishment of debt       $ 0   $ 0 5,488,000 $ 0        
Debt equity component before tax         13,000,000.0              
Debt equity component net of tax         10,649,000              
Convertible Notes 2020                        
Debt Instrument [Line Items]                        
Unamortized debt discount and issuance costs         $ 3,300,000              
Convertible Debt                        
Debt Instrument [Line Items]                        
Aggregate principal amount       494,909,000     494,909,000     $ 316,000,000    
Net of tax debt issuance cost       6,243,000     6,243,000          
Convertible Debt | Convertible Notes 2024                        
Debt Instrument [Line Items]                        
Aggregate principal amount $ 400,000,000.0     400,000,000     400,000,000          
Debt instrument interest rate, stated percentage 2.00%                      
Transaction fees and offering expenses $ 391,600,000                      
Initial conversion rate 12.3018                      
Debt conversion converted instrument amount $ 1,000           1,000          
Initial conversion price (in usd per share) | $ / shares $ 81.29                      
Conversion price percentage 130.00%                      
Stock price trigger (in usd per share) | $ / shares $ 105.68                      
Convertible threshold trading days | day 20                      
Convertible threshold consecutive trading days | day 30                      
Repurchase price percentage 100.00%                      
Convertible note hedge (in shares) | shares   4.9 4.9                  
Anti-dilution adjustments of common stock (in shares) | shares   4.9 4.9                  
Debt instrument carrying amount of liability component $ 328,000,000.0                      
Debt instrument carrying amount of equity component 72,000,000.0                      
Equity component of the 2020 Notes, net of tax $ 56,900,000                      
Effective interest rate 6.25%                      
Debt issuance directly related costs $ 8,400,000                      
Debt issuance costs 6,400,000                      
Debt issuance costs equity component 1,900,000                      
Net of tax debt issuance cost $ 1,600,000     6,069,000     6,069,000          
Convertible Debt | Convertible Notes 2020                        
Debt Instrument [Line Items]                        
Aggregate principal amount       $ 94,909,000     $ 94,909,000     316,000,000 $ 316,000,000.0  
Debt instrument interest rate, stated percentage 1.50%                   1.50%  
Initial conversion rate             14.1153          
Debt conversion converted instrument amount             $ 1,000          
Initial conversion price (in usd per share) | $ / shares       $ 70.85     $ 70.85          
Conversion price percentage             130.00%          
Stock price trigger (in usd per share) | $ / shares             $ 92.11          
Convertible threshold trading days | day             20          
Convertible threshold consecutive trading days | day             30          
Convertible note hedge (in shares) | shares                       4.4
Anti-dilution adjustments of common stock (in shares) | shares                       4.4
Initial strike price (in usd per share) | $ / shares       $ 86.59     $ 86.59          
Net of tax debt issuance cost       $ 174,000     $ 174,000     $ 1,621,000    
Debt outstanding       $ 94,900,000     $ 94,900,000