Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
Cash, Cash Equivalents And Short-Term And Long-Term Investments - Summary of Cash, Cash Equivalents and Short-Term and Long-Term Investments by Significant Investment Category (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents $ 802,771 $ 736,431 $ 802,932 $ 718,536
Short-Term Investments 87,277      
Long-Term Investments 91,086 17,396    
Adjusted Cost 981,134 753,827    
Fair Value 981,134 753,827    
Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 174,018      
Long-Term Investments 21,308      
Adjusted Cost 195,326      
Fair Value 195,326      
Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 87,277      
Long-Term Investments 69,778      
Adjusted Cost 157,055      
Fair Value 157,055      
Money market funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 159,815      
Adjusted Cost 159,815      
Fair Value 159,815      
U.S. Treasury Securities | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 14,203      
Adjusted Cost 14,203      
Fair Value 14,203      
Mutual Funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Long-Term Investments 21,308 17,396    
Adjusted Cost 21,308 17,396    
Fair Value 21,308 17,396    
Corporate Notes and Bonds [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 85,227      
Long-Term Investments 65,549      
Adjusted Cost 150,776      
Fair Value 150,776      
U.S. Agency Securities | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 2,050      
Long-Term Investments 4,229      
Adjusted Cost 6,279      
Fair Value 6,279      
Cash [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 628,753 736,431    
Fair Value $ 628,753 $ 736,431