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Cash, Cash Equivalents and Short-Term Investments - Narrative (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
investment
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 519,300,000 $ 430,000,000 $ 352,700,000 $ 356,900,000
Short-term investments 570,000,000 0    
Cash in transit for credit and debit card transactions $ 11,900,000 13,300,000    
Number of investments in unrealized loss positions | investment 202      
Other-than-temporary impairment loss $ 0      
Investment income 16,800,000 3,000,000 $ 900,000  
Cash        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 103,000,000 62,900,000    
Money market funds        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 355,500,000 $ 367,100,000