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Cash, Cash Equivalents, and Investments - Components of Cash, Cash Equivalents, and Investments (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jul. 01, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]        
Cash $ 86,005 $ 104,655    
Money market instruments 3,099 3,557    
Cash and cash equivalents 89,104 108,212 $ 129,202 $ 106,582
Current investments 364,487 427,348    
Non-current investments 408,032 262,039    
Total 861,623 797,599    
Treasury Bills        
Debt Securities, Available-for-sale [Line Items]        
Current investments 138,600 198,477    
Long-term investments 168,890 32,760    
Corporate Bonds        
Debt Securities, Available-for-sale [Line Items]        
Current investments 101,868 137,871    
Long-term investments 177,123 162,566    
Asset-Backed Securities        
Debt Securities, Available-for-sale [Line Items]        
Current investments 104,998 78,407    
Long-term investments 40,434 53,631    
Sovereign Debt [Member]        
Debt Securities, Available-for-sale [Line Items]        
Current investments 5,990 8,101    
Sovereign Bonds        
Debt Securities, Available-for-sale [Line Items]        
Long-term investments 14,833 6,316    
Agency Bonds        
Debt Securities, Available-for-sale [Line Items]        
Current investments 7,783 0    
Long-term investments 5,903 5,921    
Municipal Bonds        
Debt Securities, Available-for-sale [Line Items]        
Current investments 5,248 4,492    
Long-term investments $ 849 $ 845