Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents, Short-Term and Long-Term Investments - Summary of Cash, Cash Equivalents, Short-Term and Long-Term Investments by Significant Investment Category (Detail)

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Cash, Cash Equivalents, Short-Term and Long-Term Investments - Summary of Cash, Cash Equivalents, Short-Term and Long-Term Investments by Significant Investment Category (Detail) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Dec. 31, 2018
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents $ 1,294,222 $ 824,876 $ 824,004 $ 872,237
Short-Term Investments 106,767 112,037    
Long-Term Investments 98,698 94,589    
Adjusted Cost 1,499,687 1,031,502    
Fair Value 1,499,687 1,031,502    
Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 423,407 162,521    
Short-Term Investments 7,199 1,679    
Long-Term Investments 11,043 8,007    
Adjusted Cost 441,649 172,207    
Fair Value 441,649 172,207    
Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 99,568 110,358    
Long-Term Investments 87,655 86,582    
Adjusted Cost 187,223 196,940    
Fair Value 187,223 196,940    
Money market funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 423,407 162,521    
Adjusted Cost 423,407 162,521    
Fair Value 423,407 162,521    
U.S. Treasury Securities | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 7,199 1,679    
Long-Term Investments 11,043 8,007    
Adjusted Cost 18,242 9,686    
Fair Value 18,242 9,686    
Mutual Funds [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Long-Term Investments 34,407 28,543    
Adjusted Cost 34,407 28,543    
Fair Value 34,407 28,543    
Corporate Notes and Bonds [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 89,760 104,130    
Long-Term Investments 27,617 28,301    
Adjusted Cost 117,377 132,431    
Fair Value 117,377 132,431    
Asset-backed Securities [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 5,525 263    
Long-Term Investments 25,631 23,351    
Adjusted Cost 31,156 23,614    
Fair Value 31,156 23,614    
U.S. Agency Securities | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 4,283 5,965    
Long-Term Investments   6,387    
Adjusted Cost 4,283 12,352    
Fair Value 4,283 12,352    
Cash [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 870,815 662,355    
Fair Value $ 870,815 $ 662,355