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Convertible Notes - Narrative (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Nov. 14, 2013
USD ($)
item
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Convertible Senior Notes 0.375 Percent Due 2018 and Convertible Senior Notes 1.25 Percent Due 2020        
Convertible Notes        
Number of days within 30 consecutive trading days in which the price of the entity's common stock must exceed the conversion price for the notes to be converted 20 days      
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible 30 days      
Percentage of the closing sales price of common stock that the conversion price must exceed in order for the notes to be convertible (as a percent) 130.00%      
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period | item 5      
Number of consecutive trading days before five consecutive business days during the note measurement period | item 5      
Conversion ratio, principal amount, denominator 1,000      
Percentage of the trading price to the product of the last reported sale price of the common stock and the conversion rate, maximum (as a percent) 98.00%      
Fair value of premium stock issued and cash paid     $ 48.4  
Convertible Senior Notes 0.375 Percent Due 2018 and Convertible Senior Notes 1.25 Percent Due 2020 | Director        
Convertible Notes        
Common stock issued in exchange of notes (in shares) | shares     10,600,000  
Convertible Senior Notes 0.375 Percent Due 2018        
Convertible Notes        
Aggregate principal amount of notes $ 375.0      
Carrying value of notes     $ 7.4  
Interest rate of debt (as a percent)   0.375% 0.375% 0.375%
Conversion ratio, number of shares per principal amount   0.1932070    
Conversion ratio, principal amount, denominator   1,000    
Conversion price per share (in dollars per share) | $ / shares   $ 51.76    
Liability on notes on the date of issuance 299.4      
Equity component in convertible notes 75.6      
Debt discount and issuance costs     $ 0.3  
Aggregate principal amount of notes converted     $ 367.2  
Common stock issued in exchange of notes (in shares) | shares   148,761 7,201,058 114
Premium stock issued in exchange of notes (in shares) | shares     100,000  
Value of premium shares issued     $ 12.6  
Cash used to fund redemption of notes     2.0  
Debt exchange expense   $ 1.4    
Convertible Senior Notes 0.375 Percent Due 2018 | Director        
Convertible Notes        
Aggregate principal amount of notes 250.0      
Carrying value of notes       $ 259.0
Aggregate principal amount of notes converted     259.0  
Convertible Senior Notes 1.25 Percent Due 2020        
Convertible Notes        
Aggregate principal amount of notes 375.0      
Carrying value of notes   $ 17.4 $ 16.6  
Interest rate of debt (as a percent)   1.25% 1.25% 1.25%
Conversion ratio, number of shares per principal amount   0.1932070    
Conversion ratio, principal amount, denominator   1,000    
Conversion price per share (in dollars per share) | $ / shares   $ 51.76    
Repurchase price as a percentage of principal   100.00%    
Liability on notes on the date of issuance 274.8      
Equity component in convertible notes 100.2      
Debt discount and issuance costs   $ 1.7 $ 2.5  
Aggregate principal amount of notes converted     $ 355.6  
Common stock issued in exchange of notes (in shares) | shares     7,095,350 77
Premium stock issued in exchange of notes (in shares) | shares     200,000  
Value of premium shares issued     $ 26.8  
Cash used to fund redemption of notes     7.0  
Debt exchange expense   $ 5.1    
Convertible Senior Notes 1.25 Percent Due 2020 | Director        
Convertible Notes        
Aggregate principal amount of notes $ 250.0      
Carrying value of notes       $ 274.5
Aggregate principal amount of notes converted     $ 274.5