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Debt - Narrative (Details) - USD ($)
1 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Oct. 31, 2011
Debt Instrument [Line Items]          
Repayments of convertible senior notes   $ 0 $ 0 $ 12,900,000  
Reduction of equity component of convertible notes       47,800,000  
Cost of Treasury stock shares received   80,200,000   0  
Payments in connection with reduction of outstanding warrants   0 $ 0 643,400,000  
Warrants outstanding       0  
Borrowings outstanding $ 0 $ 0      
November 2016 Agreement [Member] [Member]          
Debt Instrument [Line Items]          
Payments in connection with reduction of outstanding warrants       401,200,000  
Warrant          
Debt Instrument [Line Items]          
Payments in connection with reduction of outstanding warrants       242,200,000  
Convertible senior notes          
Debt Instrument [Line Items]          
Aggregate principal amount         $ 400,000,000.0
Interest rate, stated percentage         1.875%
Initial conversion price (in dollars per share)         $ 84.02
Repayments of convertible senior notes       $ 12,900,000  
Stock issued, new shares (in shares)       121,058  
Reduction of equity component of convertible notes       $ (47,800,000)  
Treasury Stock          
Debt Instrument [Line Items]          
Treasury stock, shares acquired (in shares)   200,000   200,000  
Cost of Treasury stock shares received   $ 80,200,000   $ 10,100,000  
Treasury Stock | Convertible senior notes          
Debt Instrument [Line Items]          
Treasury stock, shares acquired (in shares)       121,048  
Cost of Treasury stock shares received       $ 10,200,000  
Senior unsecured revolving credit facility | Line of credit          
Debt Instrument [Line Items]          
Maximum borrowing capacity $ 750,000,000.0 750,000,000.0      
Debt term 5 years        
Increase Limit $ 250,000,000.0 250,000,000.0      
Letters of credit | Line of credit          
Debt Instrument [Line Items]          
Maximum borrowing capacity $ 50,000,000.0 $ 50,000,000.0