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Cash Equivalents and Investments - Narrative (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Restricted Cash and Cash Equivalents Items [Line Items]      
Total investments In unrealized loss position | Investment 490    
OTTI, associated with credit losses $ 0    
Available-for-sale debt securities, gross realized gain 0 $ 0 $ 0
Available-for-sale debt securities, gross realized losses 0 0 0
Equity investments, unrealized gains (losses) (3,200,000)    
Equity investments, unrealized gains   0 0
Equity investments, unrealized losses   0 0
Restricted cash and investments 52,700,000    
Prepaid expenses and other current assets      
Restricted Cash and Cash Equivalents Items [Line Items]      
Restricted cash and investments 27,600,000    
Other long-term assets      
Restricted Cash and Cash Equivalents Items [Line Items]      
Restricted cash and investments 25,100,000    
Debt Securities      
Restricted Cash and Cash Equivalents Items [Line Items]      
OTTI, associated with credit losses $ 0 $ 0 $ 0