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Cash Equivalents and Investments - Schedule of Investments (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Available-for-sale [Line Items]    
Short-Term $ 86,223 $ 21,923
Long-Term 69,699
Municipal Debt Securities [Member]    
Available-for-sale [Line Items]    
Short-Term 45,111 18,473
Long-Term 30,841
Federal Home Loan Securities [Member]    
Available-for-sale [Line Items]    
Short-Term 35,234 2,000
Long-Term 34,393
Certificates of Deposit [Member]    
Available-for-sale [Line Items]    
Short-Term 5,878 1,450
Long-Term $ 4,465