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, 2019
Dec. 31, 2018
Sep. 30, 2018
Dec. 31, 2017
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents $ 824,004 $ 872,237 $ 802,771 $ 736,431
Short-Term Investments 106,747 100,029    
Long-Term Investments 90,849 93,745    
Adjusted Cost 1,021,600 1,066,011    
Fair Value 1,021,600 1,066,011    
Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 165,745 158,613    
Short-Term Investments 1,855 4,979    
Long-Term Investments 8,094 1,976    
Adjusted Cost 175,694 165,568    
Fair Value 175,694 165,568    
Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 104,892 95,050    
Long-Term Investments 82,755 91,769    
Adjusted Cost 187,647 186,819    
Fair Value 187,647 186,819    
Money market funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 165,745 158,613    
Adjusted Cost 165,745 158,613    
Fair Value 165,745 158,613    
U.S. Treasury Securities | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 1,855 4,979    
Long-Term Investments 8,094 1,976    
Adjusted Cost 9,949 6,955    
Fair Value 9,949 6,955    
Mutual Funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Long-Term Investments 24,453 20,957    
Adjusted Cost 24,453 20,957    
Fair Value 24,453 20,957    
Corporate Notes and Bonds [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 96,899 88,412    
Long-Term Investments 31,643 43,868    
Adjusted Cost 128,542 132,280    
Fair Value 128,542 132,280    
Asset-backed Securities [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 1,372 2,115    
Long-Term Investments 19,515 21,195    
Adjusted Cost 20,887 23,310    
Fair Value 20,887 23,310    
U.S. Agency Securities | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 6,621 4,523    
Long-Term Investments 7,144 5,749    
Adjusted Cost 13,765 10,272    
Fair Value 13,765 10,272    
Cash [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 658,259 713,624    
Fair Value $ 658,259 $ 713,624