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Investments (Detail) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Cash and cash equivalents:        
AMORTIZED COST $ 234,954 $ 197,569 $ 194,078 $ 197,945
FAIR VALUE 234,954 197,569    
Investments:        
AMORTIZED COST 1,001 52,114    
GROSS UNREALIZED GAINS 0 0    
GROSS UNREALIZED LOSSES (1) (28)    
FAIR VALUE 1,000 52,086    
Total cash, cash equivalents and investments, amortized cost 235,955 249,683    
Total cash, cash equivalents and investments, fair value 235,954 249,655    
Commercial paper        
Investments:        
AMORTIZED COST   30,883    
GROSS UNREALIZED GAINS   0    
GROSS UNREALIZED LOSSES   0    
FAIR VALUE   30,883    
Corporate bonds        
Investments:        
AMORTIZED COST 1,001 21,231    
GROSS UNREALIZED GAINS 0 0    
GROSS UNREALIZED LOSSES (1) (28)    
FAIR VALUE 1,000 21,203    
Cash and money market funds        
Cash and cash equivalents:        
AMORTIZED COST 234,954 197,569    
FAIR VALUE $ 234,954 $ 197,569