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)

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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
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents $ 844,847 $ 736,431 $ 751,581 $ 718,536
Short-Term Investments 42,895      
Long-Term Investments 23,954 17,396    
Adjusted Cost 911,696 753,827    
Fair Value 911,696 753,827    
Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 102,883      
Long-Term Investments 19,822      
Adjusted Cost 122,705      
Fair Value 122,705      
Money market funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 102,883      
Fair Value 102,883      
Mutual Funds [Member] | Level 1 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Long-Term Investments 19,822 17,396    
Adjusted Cost 19,822 17,396    
Fair Value 19,822 17,396    
Corporate Notes and Bonds [Member] | Level 2 [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Short-Term Investments 42,895      
Long-Term Investments 4,132      
Adjusted Cost 47,027      
Fair Value 47,027      
Cash [Member]        
Cash Cash Equivalents Short Term And Long Term Investments [Line Items]        
Cash and cash equivalents 741,964 736,431    
Fair Value $ 741,964 $ 736,431