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Investments - Additional Information (Detail)
3 Months Ended
Mar. 31, 2019
USD ($)
security
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Debt Securities, Available-for-sale [Abstract]      
Available-for-sale securities maturities term maximum 36 months    
Number of available-for-sale securities in unrealized loss position | security 239    
Other than temporary impairment losses, investments, available-for-sale securities $ 0    
Unrealized gains relating to the changes in fair value   $ 18,500,000  
Marketable equity securities 0   $ 0
Equity investments in privately-held companies $ 14,900,000   $ 14,600,000