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Long-Term Investments - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Unrealized Losses On Short Term And Long Term Investments [Line Items]        
Minimum period for which failed auction securities been in unrealized loss position 12 months      
Failed Auction Security [Member]        
Unrealized Losses On Short Term And Long Term Investments [Line Items]        
Amortized cost of securities $ 3,000,000 $ 3,000,000    
Period for which failed auction securities been in unrealized loss position exceeds 12 months      
Estimated Fair Value $ 2,526,000 2,525,000    
Gross Unrealized Losses 474,000 475,000    
Aggregate credit loss 41,000 $ 48,000 $ 59,000 $ 72,000
Aggregate temporary impairment loss $ 433,000