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Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule Of Investments [Line Items]    
Investments $ 48,915  
Investments 757,233 $ 725,227
Commercial paper    
Schedule Of Investments [Line Items]    
Investments 135,197 94,572
Corporate debt securities    
Schedule Of Investments [Line Items]    
Investments 414,951 544,978
Securities of government-sponsored entities    
Schedule Of Investments [Line Items]    
Investments 158,170 $ 85,677
Restricted equity securities    
Schedule Of Investments [Line Items]    
Investments $ 48,915