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Cash, Cash Equivalents and Short-Term Investments - Narrative (Details)
3 Months Ended
Mar. 31, 2019
USD ($)
investment
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Cash and Cash Equivalents [Abstract]      
Cash in transit for credit and debit card transactions $ 15,800,000   $ 11,900,000
Number of investments in unrealized loss positions | investment 65    
Other-than-temporary impairment loss $ 0    
Interest income $ 6,200,000 $ 1,400,000